Hey guys this is my first guide so bare with me and I'm willing to take any suggestions on improving or editing this guide :)

I mostly play AD/AP Assassin's and this guy really got me excited when I first heard about him since I've been maining (Talon) (and Akali but she's always banned...)

This champion I found out is extremely well duo lane or solo lane whether it be top or bottom, this guide specifically talks about SOLO TOP
and SOLO MID

TWO BUILDS -
The first is if you are experienced with this champion and has very little survivability and relies on your expertise of the game and knowing the fundamentals downpat.

The second is a more beginnerish (not calling anyone a noob) build if your somewhat new to the champion and want to get a feel for how he works.

Breakdown

Pros -
~Extremely annoying harasser in lane
~Can avoid/dodge enemy nukes/CC
~Has RIDICULOUS burst mid-late game

Cons -
~Pretty squishy until Rod of Ages
~First priority target after Playful/Trickster
~Melee AA for an AP Carry

I chose a 21/0/9 build that has been working out amazingly for me. The short and simple version of this is pure AP with Utility for Mana Regen with 20% from shrines and buff durations.

Breakdown
1/1 Summoner's Wrath - Both Summoner Spells you are running are viable and are in this talent point. Take it.
4/4 Mental Force - MOAR STARTING AP! YES!
4/4 Sorcery - More CDR on this champion HELPS, but is not truely needed as to where you would take runes over AP/AD
1/1 Arcane Knowledge - Um. 10% Magic Pen for 1 point. Yes please.
3/3 Havoc - 1.5% increased overall damage? YES.
3/4 Blast - More AP per level, yes. Would of put 1 more but wanted the last point in Utility for shrines/relic buff durations increased.
4/4 Archmage - ...Built in mini Rabadon's Passive? .... yes. lol.
1/1 Executioner - Not only this being the 'ultimate' point in this tree, it synergizes so well with this champion's harassing/bruising ability early game.

3/3 Expanded Mind - Mana per level? OFC. Helping this champions small mana issues balances him out.
1/3 Good Hands - Point filler, plus if your dead, helps to recover alittle faster. :)
3/3 Meditation - Yet again, same thing with Expanded Mind.
1/4 Swiftness - After debating between this and Scout, decided to go with this since you can move through creeps early game and not get creep blocked, alittle extra speed helps, plus your not really going to be supporting your team heavily since you are not a support therefore you will not be warding everywhere, maybe only a couple here and there
1/1 Runic Affinity - You probably wont be taking blue/red that often, but trust me, you will acquire it through killing enemies that have the buff. And, increased Baron Buff is nice as well.

Advanced Build (1)

I choose to play more aggressive and take a less health/oriented build as the only item I get that's health related is Rod of Ages until the very end game where you see Rylai's paired with Void Staff. In some cases, if it's a generally balanced game and we are in the late game and I have to chose between a Rylai's and a Void Staff, it depends on how much MR the enemy team is building against me and my team.

I like to rush Rabadon's Deathcap after the Sheen, sometimes, depends on how fed I am I will get the Deathcap before a Sheen. Rabadon's Deathcap + Lich Bane = Destruction....it seriously is so overpowered you will wonder why the hell Riot made a champion that would benefit 10x more than any other champion would from these two items alone..

But wait, Kassadin is an AP Carry, that is an Assassin, and does melee damage...wtf? Fizz actually benefits more from auto attacks than Kassadin does, such as his Seastone Trident on activation increases his attack damage and dots them for a percentage of their health. I have seriously 2 shotted AP casters from Urchin Strike + Seastone Trident. No lie.

Beginner/Starter Build

This starter build focuses more on survivability with alittle less burst. If your new to this champion or new to the game in general try this build out if you are dying more and can't get fed.

DORANS RINGS

My take on Dorans Ring for this champion is like any other. If you are 0-3 Mid or Top lane you should BUY THIS ITEM OR EVEN STACK THEM Doran's items to me are a fallback if you aren't doing so hot. Don't think of them as a waste because they don't go into anything, think of them as a boost to your overall (early/mid) game. 1 - 2 Doran's is worth killing your 3-0 killing spree opponent top or mid. 238 from selling 1 ring + 299 from you killing your opponent top makes that doran's ring worth it. :)

Mejai's Soulstealer

If properly fed with this item the enemy team will surrender at 20 and you will be ~10-2 with 15-20 stacks. If I feel the enemy team isn't as good as I am or I'm doing good sometimes I'll rush it right after catalyst, skip the Rabadon's, and go right for a Lich Bane. Just remember though...you will be priority #1 so don't wander out alone...

Your primary skill sequence varies depending on how far away you are from the champion before they see you and back up or if you ambush them. Always activate your Seastone Trident before starting your attacks; what you want to do is either Urchin Strike to them, hit them with an auto attack with Seastone Trident activated, then use Playful/Trickster to jump to where they are fleeing to, then Chum the Waters/Ignite should take care of them.

If ambushing you could use just Playful for the slow in the brush as they run into it, slowing them; activate your W and pound on them as they run away. If they ghost or Flash or manage to get some distance on you, Urchin Strike in, AA again with Seastone Trident activated, Chum the Waters/Ignite and this should do the job.

Team Work/Fights

Have your tank or initator (like they always do) start the team fight. Activate your Seastone Trident and then use your Urchin Strike to dash to an AP Carry or AD carry standing behind the rest of their team. Use your Playful/Trickster ONLY to get away, the first game I played him in being aggressive in team fights using Playful/Trickster early basically painted a blood red target on me and I was focused hard. Use this skill as a utility to get away fast, then Urchin Strike back in with an activated Seastone Trident.

Now (some of you may disagree with me on this but it's been working for me) during teamfights I use Chum the Waters in different ways; if we are losing the team fight, tag an enemy champion with it to push them or the rest of their team away. If we are generally winning the team fight, I will use it to net kills or to significantly damage the other team.

Chum the Waters can be a great out and a great baiting tool since it slows for your team to respond to those few greedy enough trying to net a kill off of you in a gank.

Just use your gamesense :)

Like I said this is my first guide so please don't bash it, any tips would be greatly appreciated/added but this is how I play this little guy and it works out for meh ;) I will be adding more in the future, just running out of time and I've got to jet. Thanks for checking it out :)