Your E is a awesome ability! You can avoid Karths ult, everyones! Just press E and he jumps up on hes little staff and he will be invurnable! And your W gives you passive damage! So fizz is a nice Poker, cause he can jump on (Q) Hit with a normal atk(Passive W) and then use your E to jump away, and if you got a good jungler he should give you the blue buff. then you can like spamm all your spells! And if you throw a shark and do your combo (Q) (W) (E) they should be on very low health, and your ignite with W Deals alot of damage over time!
If they stack magic resist you can buy extra magic penetration! Just a tip ;)

When youre in the middle you normaly play against a ap carry. Thats why I added alot of magic resist, but if you want to solo top I recomend Armor! Alot of seal of warding cause Fizz deal alot of damage with spells! And magic pene so he can deal extra damage.