Is Fizz impossible to kite?

I'd say yes to that question. Even as a squishy mid-laner even ONE kill can lead to the game spiraling downhill for the rest of your team especially for your squishy supp or ad carry

The best counter for Fizz is a ban. Second best is an afk. Hold on, you might thinking, "Afk ? how can I be such a dick to my team ?". Thing is, you won't feed him 12 kills by 15 minutes if you afk and your team would actually have a chance at winning.

unless someone can destroy him and burst him in a 1v1 scenario. also being tank enough to sustain his burst. is a solution against fizz but he can still one shot the squishies so depending on the skill he can almost always get a kill.

Alone? Yeah, probably. That's the point. If you could just kite assassins 1v1 as a mage or adc after early game, there'd be no reason to pick assassins.