Fizz ability is very strong so he have huge AP casted. But in late game in team fights if you used your skills (Q W E R) to kill for example adc you will die becous you are not enought tanky. This build is effective in early game becous of the same dmg like old fizz and fevor. In mid game you can moving aroud the map and kiling enemy becous you are tanky and still do dmg. in late game you can think that his W wont do dmg becous you lost somethink aroun 165 AP per aa (33% of 500AP)
Nothing could be more wrong :) this build gives you lots of AS so you can hiting faster and you have more than 3000 hp and your W deal (+ 4/4.5/5/5.5/6% of missing health) so if you have 50% HP all your aa will deal + 90dmg if 33% all your aa will deal + 120dmg. That why I show you this build. Dont forget, if you have to choose between HP and (MR or Armor) chose HP.

Interesting, but now his W is just a plain AP casted ability, can you explain how this build can be effective?