I'll start of by saying 2 things:
1. This is my first build/guide for LoL (or any game for that matter).
2. As with all guides, please don't rank until you've tried it a few times.

As this is my first guide, I'd like to thank jhoijhoi for her guide-writing guide, which you can find here. It's very helpful and saved me loads of time.

I've tried to make this guide as in depth as possible, yet not so lengthy that you couldn't stand reading it. If this seems a bit long for you, jump to the Tips/Tricks and summary sections for a quick overview. However, you will be missing out on quite a bit if you do.

I know some of you might be a bit annoyed by my rune and mastery choices (why doesn't he focus on pure damage???). However, since most games I play are randomly matched, you usually get bad teamwork and mismatched champs ("No! We are premade, so both of us go top even though we are both ap carry. Alistar can take mid while Zilean jungles!"). Therefore, this build allows you to have decent damage output while not relying too heavily on your team in most cases, and allowing you to be more durable early game when you most need it. In fact, you are surprisingly durable and most players won't expect that and try focusing you for first blood - this can make them waste their CD and summoner spells and allow you and your lane partner to get first blood.

After the initial durability boost, I focus on AP/AS to allow sustained damage output, while taking CD reduction for more efficient burst damage and quick getaways when necessary. AS also helps with farming creeps and taking Lizard/Golem buffs when possible.

In general, remember one thing: you are mainly an assassin champ. Join in mid-fights with burst damage (then get out) or chase down a fleeing enemy. While you will be able to hold 1v1, do not try to initiate large fights or chase 2 enemies at once.

Pros: High damage output Can evade dangerous champion ultis Most players underestimate DoT A giant shark eats your foes

Cons: Hard to master High CD for burst champ Weak farmer without high AS Allies underestimate your DoT as well = you get KSed

The DoT (damage over time, in case I haven't mentioned that yet) is also a Pro/Con in and of itself: while fleeing enemies tend to die on the run, the ones attacking you have a few extra seconds to hit you before they die.

Once again, focusing on defense seals and glyphs( Greater Glyph of Magic Resist and greater seal of defense), the reason for which I will not explain a third time :)
Greater Quintessence of Ability Power is probably my only "obvious" choice for Fizz, to give him some starting power. For marks I take Greater Mark of Attack Speed. You are free to take Greater Mark of Magic Penetration, however I believe the 15% increase in attack speed is much more helpful for applying your on hit effect as well as to improve farming (which is a problem for Fizz). I'll explain the farming problem in detail in the relevant chapter.

This is one of the most important aspects of the guide, as Fizz can be a bit item dependent. Since my runes and masteries are based on durability, I can forgo the Doran's Ring and start with an Amplifying Tome for added ability power and to rush Kage's Lucky Pick. This also leaves me some extra gold for a Health Potion. Afterwards get Boots quickly, since Fizz is a bit slow. Fiendish Codex is your next buy for the CD reduction, ability power and mana regeneration before you get Sorcerer's Shoes. Build into Deathfire Grasp and get Malady, which are your main items, and now you're good to go.

I will explain the need for, and use of, Deathfire Grasp and Malady before continuing to the rest of the item sequence:
Deathfire Grasp improves your ability CD as well as adding AP and mana regen, however its main feature is the "60 second cooldown - ranged - 30% (+3.5% per 100 AP) of current enemy health" magic damage. Use this as often as you can, as it does 2 things:

1. Causes a great deal of damage
2. Scares enemy players when they see a huge chunk of health loss (psychological effect)

How to best use Deathfire Grasp? With your high attack speed, try to get in 3-4 hits on the enemy. With Malady, their magic resistance is now 18-24 points lower. Now use Urchin Strike to get distance and hit them with Deathfire Grasp. Use Playful / Trickster so they can't target you with anything (this really annoys players), and then again to cause more damage and get close to them if they started running. By now, with their missing health, Seastone Trident should do quite a bit of damage and they will start running away. Chase and kill if you can.

On to the rest of the items:
Sheen into Lich Bane for some on hit damage (don't forget that Urchin Strike triggers on hit effects). Get Nashor's Tooth for more AS, AP and to round out your CD reduction to 40%. For the end-game, Rabadon's Deathcap increases your AP to give great burst and on-hit damage.

As always, nothing is written in stone. If you find yourself in tight spots needing a little more MR, you can take Abyssal Mask instead of Lich Bane. If you need some armor and also seem to be doing well in gold, you can take Madred's Razors to build into Madred's Bloodrazor later on (great add-on for your on-hit damage). However, try not to deviate too much from the original build as the items are meant to work with each other and with your abilities.

Nimble Fighter - Fizz's dexterity allows him to ignore unit collision and take (4/6/8/10/12/14) less physical damage from basic attacks.

Ever get blocked by your pesky minions after turret diving, only to get killed by a turret shot you could have otherwise escaped? Not anymore! Fizz just dashes through those minions as if they were clean air. This is very helpful in turret dive escapes as well as escaping foes who must run around the minions. The second part of the passive is also very helpful to your survival, mainly helpful against focused minion attacks. Also, as enemies underestimate your durability they may try chasing you into a turret thinking they can kill you with a quick hit and get away, only to die to the turret and your DoT. Of course, remember that your are not invincible, it'll just take an extra auto attack or two to take you out.

Urchin Strike - Fizz dashes through his target, dealing normal attack damage plus 10/40/70/100/130 (+0.7) magic damage. This ability can trigger on-hit effects.

Your attack and retreat initiator. The way this ability works is that you target and enemy, and Fizz dashes a set distance in the direction of that enemy while damaging them. This means that if you are far from the enemy you will dash into him, but if you are close you will dash through and away from them a set distance. This allows you to gain a head start when retreating. Note that if you auto attacked and then used the ability, Fizz will run back to attack the enemy after dashing away - so remember to click move right after dashing to cancel the auto attack when retreating.
An added bonus is that this applies on-hit effects, including Seastone Trident.

Seastone Trident - Fizz's Trident causes rending wounds in his opponents, dealing magic damage to the target based on their missing health.

Passive: Fizz's attacks rend his enemies, dealing 30/40/50/60/70 (+0.35) plus 4/5/6/7/8% of the target's missing health as magic damage over 3 seconds. (Max 300 vs. minions)

Active: For 5 seconds, Fizz's attacks deal an additional 10/20/30/40/50 (+0.35) magic damage on hit and apply Grevious Wounds to his target, reducing incoming healing by 50%.

Your "bread and butter" so to speak, with two wonderful effects: additional damage and healing reduction. What's great about this skill is that the passive remains while the active is on CD. This means that when active, your attacks do quite a bit of damage, as well as reduce healing (which is great against those pesky opponents who take 3 health potions at level 1 and just won't die!). The active lasts for 5 seconds and has a 10 second CD on all levels (lowered down to 6 seconds with max CD reduction - which means almost always on). This is another reason for AS, to utilize the active property as much as possible in 5 secs.

The passive gives nice bonus damage as DoT, which grows with declining enemy health. This can stack up quite nicely with the added AP bonus and cause some serious damage that the enemy doesn't notice until 3 seconds later when it's too late. The DoT doesn't stack, it only renews with each hit. This means that if you attack for 3 seconds, hitting 5 times, you only get 6 seconds worth of passive damage (3 during the attack and 3 from the last hit). However, each attack triggers more bonus damage because it is based on missing health.

There are three things that should be noted here:

1. Since the added damage is on-hit magic damage, the bonus damage is not returned by Thornmail.

2. Since it is on-hit magic damage, if you are blinded by an ability such as Blinding Dart, or if your attack is dodged - the attack is considered a miss and the bonus damage is not applied either.

3. Remember all those times you placed Ignite on a fleeing enemy, knowing they will die, but then one of your teammates chases and picks up the kill? This happens even more often with Seastone Trident. There have been many times that I've let an enemy run to his turret without chasing because I know he will die to my DoT, just to have a teammate Flash in, take the kill and run away (or even die to the turret). This gets very annoying, so try explaining your skills to teammates unfamiliar with your abilities.

Playful / Trickster - Fizz hops into the air, landing gracefully upon his spear becoming untargetable. From this position, Fizz can either slam the ground or choose to jump again before smashing back down.

Fizz hops toward your cursor onto his spear, becoming untargetable, and then slams the ground dealing 70/120/170/220/270 (+0.75) magic damage to nearby enemies and slowing them by 40/45/50/55/60% for 2 seconds.

Reactiving this ability while Fizz is on top of his spear, causes Fizz to jump again towards your cursor and deal 70/120/170/220/270 (+0.75) magic damage in a smaller area to enemies nearby, instead of slamming the ground.

Many champs have that one skill that really annoys anyone who plays against them. Twitch has Ambush, Tryndamere has Undying Rage, Wukong has Cyclone and so on. Fizz has Playful / Trickster. This ability allows for chases and getaways, as well as causing lots of AoE damage at high levels. The ability is used like Flash - place the cursor in the direction you want to jump and press [e]. Then you can wait and do AoE damage in half a second, or jump again in the direction of the cursor by pressing [e], doing AoE damage at the target location. While the distance of each jump is quite short, a double jump can greatly assist in chasing an enemy. You can also use this to jump over thin walls.
Now, why is this ability so annoying? Not only do you jump and deal damage, but on the first jump you become untargetable for half a second. This may not sound like a lot, however:

1. it can be used after a turret dive to avoid being hit by the turret.
2. it clears minion aggro after attacking a champion.
3. it can be used to escape channeled spells (!!!)

Number 3 is the best part of this skill. I have used Playful / Trickster to escape Bountiful Harvest, Ace in the Hole, Requiem and many other abilities that would otherwise have killed me (note that Requiem escapes must be timed precisely as you are only untargetable for a short time).
This ability has only one flaw - its cooldown is quite high (16/14/12/10/8). This is why you must have max CD reduction for Fizz.

Chum the Waters - Fizz unleashes a magical fish that latches onto enemies or hovers over terrain, slowing champions if it is latched on to them. After a brief delay, a shark erupts from beneath the earth, dealing damage to enemies around the fish and knocking them aside.

Fizz releases a magical fish toward a location that can bind itself onto enemy champions, slowing their movement by 50/60/70%.

After 1.5 seconds, a shark erupts from the earth, dealing 200/325/450 (+1) magic damage in a large area, knocking up the target the fish is attached to, knocking other enemies aside and reducing the movement speed of all enemies hit by 50/60/70% for 1.5 seconds.

SHAAAAARK!! is Fizz's war cry. This ability is your all-in-one CC and nuke, slowing and dealing great damage. It is cast as a skillshot, meaning you have to practice a bit to learn its speed and behavior before mastering it. Basically, you throw a fish wish either latches on to champion and slows them, or lands on the ground and latches on to a passing champion. In either case, after 1.5 seconds - shark attack :) Other than just looking cool, the shark knocks up the main target, knocks back other enemies and deals AoE damage scaled 1 for 1 on AP. As opposed to Fizz's other skill, the CD is quite low for an ulti (100,85,70), reaching 42 seconds with max CD reduction, which makes this a very versatile skill:

• Use to slow for 3 seconds + knock up a fleeing (or chasing) enemy (this has saved me or a teammate quite a few times).
• Use in mid team fights to break up clustered enemies, or in late team fights to finish off low health enemies.
• Use to separate an enemy from the rest of the group if they are fleeing.
• Use as an initiator when ganking.
• Use along with the Deathfire Grasp + Malady combo explained earlier to deal insane damage to a single target and finish them off.

If timed correctly, Chum the Waters is a very powerful ability.

The ability sequence is quite clear in my opinion. Your main source of damage is the bonus damage from Seastone Trident, which I take at level 1 and max as soon as possible. Your second spell should be Playful / Trickster to help you escape dangerous situations early on (such as jungler ganks), and to clear minion waves if you've managed to push your lane opponents back enough that they aren't a risk. Take Urchin Strike next so that you have all 3 basic abilities at level 3 for the extra damage as well as an escape mechanism. From now on max abilities in the order of Chum the Waters --> Seastone Trident --> Playful / Trickster --> Urchin Strike. The only exception being that I prefer taking another level of Urchin Strike at level 12 for its CD reduction in case I haven't managed to get enough CD reduction items by then.If you find yourself not getting AS items quick enough, and getting into a lot of trouble needing your Playful / Trickster, consider leveling it alongside Seastone Trident (giving each 1 level at a time) for the added cooldown reduction and some extra AoE damage (which will help you farm as well)

I have to admit that summoner spell knowledge and use is one of my weaknesses, however I believe I've found the best fit for Fizz, at least with this build. I take Surge and Ignite which are a great damage boost, and both get buffed by Summoner's Wrath so you only need one mastery point for them. Feel free to experiment here.

My Favorites

Surge is a powerful spell, and well fitted for any build based on AP/AS, such as this one. With the Summoner's Wrath bonus, it's very helpful in getting first blood, especially when used along with Ignite. Also, its 12 second duration makes it a very high duration buff (not many in the game). Since summoner spell casts are seen by other players, and they may start running when you cast, try casting Surge out of enemy sight. Since it lasts 12 seconds, you have enough time to reach and attack them before they realize you have the upper hand.

Ignite is the standard DPS assistance spell, used for fast fleeing foes. Along with your Seastone Trident passive, enemies won't notice their health disappearing before it's too late (very similar to Twitch and Teemo poisons). The added healing reduction is also very beneficial (to you).

Exhaust is another useful spell, both in assisting escapes and in assisting chases. I take this occasionally if I feel a bit off my game and think I need the extra defensive feature more than the damage Ignite gives.

Viable Options

Flash is a nice addition to your elusive abilities. Take it if you are new to Fizz and feel you may need the added mobility. Of course, it can also be used for an easy chase. Very few enemies can outrun Chum the Waters + Playful / Trickster + Flash + Urchin Strike.

Ghost basically gives you the same advantages as Flash. Each player has their own preference between these two, however I feel that since Nimble Fighter allows passing through units anyway, Ghost is partially wasted on Fizz.

Heal is also viable for the added sustainability, making it quite hard to kill Fizz 1v1, or even 2v1 if used correctly. However, since you need sumoner's resolve for the buff, it may not be worth taking as you'd need another mastery point for it. Also, many champs have recuded healing debuffs or take Ignite, so Heal will be less effective.

Teleport is useful if you tend to spam abilities and may need to return to base early for a mana recharge. This way you won't waste precious time getting back in lane. Fizz is also a viable mid lane champion, where Teleport is very handy for getting the upper hand on your opponent. In addition, Teleport is a great spell for ganking, especially if you have a champion like Teemo on your team hiding his Noxious Trap in bushes.

Do Not Take

Revive: Just not useful for an assassin champ, and you already have Good Hands for the few deaths you will encounter.

Smite: You are not a jungler. While Fizz can be played as a jungler if you really want to build towards it, I find it's not a very good position for him and this build isn't really geared towards that.

Clarity: While slightly mana hungry at low levels, Fizz doesn't need that much mana replenishment at higher levels. Deathfire graps and Nashor's Tooth provide enough MP5 (mana per 5 seconds) that you don't need Clarity.

Early to Mid Game

Farming is a problem in early levels for Fizz. It boils down to 2 issues:

• No cheap, short-cooldown AoE to take out minion groups
• Low attack damage (melee damage based on DoT)

I'll explain the latter:
Let's say we have a minion with 450 hp, whose health is now down to 100. You are at level 1 with 51 attack damage and 38 ability power. You have Seastone Trident as your first skill (not activated at the moment). This means that your attack does:

51 physical damage
30 + 0.35*38 = 43 magic damage (DoT)
4% of missing health (350 hp) = 14 magic damage (DoT)

This gives a total of 108 damage which is enough to kill that pesky minion who has only 100 hp left (this is assuming 0 armor and magic resistance of first minion wave). HOWEVER: this takes 3 seconds to happen, and in that time your "greedy" lane partner will see a minion with low health and quickly attack it - stealing your precious gold one second before the DoT finishes the job. This also tends to happen with your minions, as they don't care about the impending doom of their foe and continue attacking anyway.
So...added attack speed helps you pick up that last hit on a minion to assist your DoT.

Mid to Late Game

With better cooldowns, greater attack speed and high DoT it is now easy to clear minions waves and farm nicely. Try to pick up large waves scurrying along unprotected. In order to make the most out of your Seastone Trident passive, try hitting each minion once and then going around again. This pretty much ensures the quickest kills. By late game though you should have enough AS and AP that you just 2-hit each minion in rapid succession, and have enough CD reduction so that you can use Playful / Trickster on minions without worrying about having it ready if an enemy comes.

I've already mentioned the Deathfire Grasp + Malady combo in the items section (if you skipped here directly, I advise you go back and read it), and my preferred farming methods. These are core moves with Fizz. Here are some other general tips and tricks I've picked up along the way:

The Great Escape

The Dive

Ready...Aim...Miss!

Where'd you Come From???

Where'd he Go???

Two Seconds can Save your Life

This section will basically be a quick review of what I've said so far, for those who couldn't bother reading the whole thing (or just want a quick reminder).
So, what we have here is an assassin AP/AS champ with heavy defensive masteries and runes to start the game out with for durability. Taking Sorcerer's Shoes, Deathfire Grasp and Malady as core items which benefit Fizz's abilities greatly and give a nice one-two health chomping combo. Work up some more CD reduction, AS and AP to increase your deadliness and you're a force to be reckoned with.
Try to initiate only 1v1 or chase down fleeing foes. In team fights, join mid-way with Chum the Waters and focus on picking off one foe at a time. In case of emergency, use Urchin Strike for a quick getaway and pop Playful / Trickster to release any targeting and flee.
Beware of kill steals due to your teammates underestimating your DoT. Try to ensure a kill by last hitting, because getting KSed will cost you items in the long run.

I hope this guide was helpful to you, and please rate me if it was :)
If you find any mistakes, or have any requests for addition please tell me and I will try to keep this guide updated.
Thanks for reading! GL & HF!

Once again, I'd like to thank jhoijhoi for her guide-writing guide, which you can find here.