Hello, I'm a modest player of Lol since a lot of time, i'm not english so please be indulgent if i did mistakes in language ;)

Feel free to ask me questions and tell me if there is mistakes in this guide, give me your opinion about this :)
Guide is not finished.

I hope you will like it !

This is the runes i used in most of my games, MagicPen is useful, you will need it because as you can see on the build i didn't choose sorcerer shoes. You will need the armor in any case.
Note that you can change your MR Glyph with AP Glyps if you are vs an ad champ and want to play more agressive.

Cunning

5 point in Mercyless is really great in synergy with the passive of your W (Trident), Bandit will give you more gold along the game, intelligence permite you to have 45% CDR which is perfect with fizz.
With the core build, you'll have 45% in mid game, what it means ?

It means less than 3.5sec for your Q, less than 6s for your W, less than 5 sec to your E.
You will be able to use your ultimate all the 38.5sec.
It means that, even if you're squishy champ, you will be very difficult to catch with your E and A and you will do crazy damage!
For the last point, I prefer stormraiders surge because it permite to Burst then replace fast, useful to escape too.
But thunderlords decree is viable too, your burst will be greater.

Ferocity

I up feast for sustain in lane, sorcery and natural talent for more damage and bounty hunter because you are an assassin, without slow, you could reach at least 2/3% more damage.

Pros:

High mobility when you got your core items
Very heavy burst
Strong 1v1 champ


Cons:

Need some practice to master
Squishy
Need a nice early game (and kills/assists)

If you follow the build, you will have 45% CDR, what it means a lot of escape because of your E. But, i always take a flash,the more versatile spell to my mind, useful to escape, to secure a kill...
BUT if you are really confident and use your E nicely, you can take heal or barrier, but for me it's risky.

Take ignite for having more pressure in lane and secure kills, also good vs ad carry (with heal) or a mundo.
If you think you don't need it, i recommend to take TP, nice for helping mates and ganking.
If you are afraid vs a heavy burst champ, you can take barrier, but personally i never use it.

Start build

I often start with dorans ring and 2 pots, for me it's the more versatile item for start with all you need.
If you are vs a hard ad champ, you can replace it by cloth armor and rush the seeker's armguard.

Core build

Next your start build you can go for boots of speed, then, i prefer to buy a sheen and rush Morellonomicon because it's a very cheap and effective item. The passive is also great when you shift to botlane, you will decrease the heal.
When you have your Morellonomicon, you can finish your Lish bane.
Next let's finish your boots of Lucidity

Why boots of Lucidity and not sorcerers shoes ?

Because i think to have 45% CDR is a more versatile option.
Sorcerer will give you damages, that's nice, but that's all. With boots of lucidity (and 45%cdr) you will do less damages, but you could use your spells more often, and more spells = more skill = more combos = more escapes/mobility. Moreover, you will have a 10% reduction to your spells (Flash..) So i think it's a good choice, if you really need MPen, you can buy Void staff or Lyandri's.

Now you have 45% CDR, now you will need the Zhonya
because you're squishy, and the passive will allow you to burst and protect from focus. If you are in bad situation (CC, stuns...) use it, know you have your E all the 4.5 sec (approximately). The time your zhonya is over, you will be able to do your E again for run or re-burst.

Late game items

Now, you have two slots free, you will choose your late items depending on the ennemy team.

VS strong tanks: --> Void staff, Lyandry's (MPen)
VS strong AP: --> Abyssal scepter (always viable with 20 MPen!)
For more mobility --> Luden's mobility is great, but i don't think the passive of Luden's is the best option for Fizz.
For more damage --> Rabadon
Survability, synergy with zhonya --> Guardian angel

I recommended to max your E as soon as possible, very important for escape to a gank or chase an ennemy, this skill has also a nice ratio with your Ap and will do good damages.
Then there is two possiblities:

-->You want to increase your damages: if you choose to max your damages, up your W before your Q
-->You want to harass more often: Up your Q first, you could harass more with Q+W and then go back with your E, i think it's a nice strategy if you are against strong players and want to play safer. It also give you more mobility.

If you won't choose, you can up alternatively your Q and W, it's a good alternative.

How to use your skills:

W+Q, then E for chasing and slow, or for go back.
Look to your ennemy hp, the lowest he is, and the heaviest damage you will do. Try to abuse him with this combo and go back with E. When your opponent is low/middle life, wait your ultimate, then:

R+W+Q+E, in most situations, your versus will die, if you are not sure, you can use your ignite for secure the kill.

Urchin strike permit to cross too, only if you have vision on the other side.

Playful/trickster: be careful of Lux, ziggs, zed..ect ultis ! with your E you can avoid these spells !
Important to know that you can cross walls (not the biggest) with your E. If you are close to a wall, you can use your E for going to the other side, then re-use it for going back, it permite to have vision without risks, except if you failed...

Last thing, if you are chased, in a really bad situation, try to go to the nearest bush then stop. When the ennemy is going to enter in the bush, use your Q on him and then your E, you will gain distance and save your life !