If you've ever played against a fizz in mid you'll know the pain of that slippery little bastard bouncing out of the way of your perfectly aimed skillshot combo and him walking out with no damage and half of your health bar. The ability is objectively far too overpowered, even if you ignore the rest of his kit.
Fizz's Flip renders him Untargetable, Invulnerable, allows him to close distance, and does damage with no drawbacks. Each of these status's/effects are powerful enough to be considered ults on some characters. Kayle, Twitch, Rengar, Khazix, Ekko, Rek'sai, Kai'sa, Xayah, Nocturne, Etc. have two maybe 3 of these effects as their ULTIMATES, Ekko being the only one with all 4. Fizz having all that as an ability with a 12 second CD is waaaaaaaaaaaay too overpowered no mater how you look at it. The ability has no drawbacks to use that aren't just basic problems every character has (Like CD or Mana consumption) and does far too much considering the rest of his kits damage.
Everyone here has seen a fizz throw his ult and flip simultaneously and destroy your HP in 0.0 seconds. I have an actual screenshot of him doing 2.3K in 0.0 on a match i wasn't behind on.
The reason i mention his Untargetable status with his ult because it renders him invincible from tower damage as well. This one part of his kit is the root of the problem and it cant be fixed by adjusting damage, Mana, duration or CD. Its problem is the Invaun status when he uses it, The fact hes completely invincible in the two seconds hes moving makes it too OP.