I take all AP runes because the more damage the faster you can kill the enemy squishy. He is an assassin after all.

The masteries I choose are for increase of damage and mobility. Don't need mana/mana regen because you shouldnt really be spamming your moves early.

The items I chose to use on Fizz give you the most nuke damage. I've done well opting to take nashor's tooth over GA for some extra damage when the tanks/tanky people are left to be able to deal more damage due to your W.

Pros
High Burst
Ability to get in and get out of combat while trying to kill their carries
Mobility
Increases damage the lower health target has.
Heal reduction debuff
Short ult cooldown

Cons
Just a burst assassin (Unless you buy nashor's tooth)
Needs to snowball or he falls off.

I take Ghost/Flash with Ignite or I run a ghost flash for summoners for more mobility/chase potential.

The skill order I use lowers your escape/chase cooldown pretty well. Only need 1 point in your W early and leveling your Q after your E gives you great nuking damage. Once you get your E maxed it is really up to you if you would rather have a more potent DoT/health shred or a greater nuke.