Tank Fizz Broken

So I played a game in the jungle on Summoners Rift in normals. I was playing Kayn and I went shadow assassin. The opponent jungler was Fizz . He ended up going around 14/0/2 by the time I left the game due to urgent business. But this Fizz was one shotting everyone besides our top who was Aatrox . He ended up building a few tank items before building Blade of the Ruined King and he still was one shotting everybody. I'm not sure how or why he is able to one shot people even though he didn't have a single shred of AP. He was unkillable and could one shot anybody which in my opinion, isn't balanced in the slightest. The Fizz was tank/bruiser and building armor and AD and was still one shotting practically everybody. Now it makes sense for the squishies such as our adc and me since I was blue Kayn . But my question is how and why is a tank Fizz able to deal that much damage? I think nerfing his base damage and giving a slight buff to his AP ratio would be good so a tank fizz can't go one shotting everyone on the enemy team. Now I do realize this will affect his early game in a negative way however, Fizz excels in the late game anyways so his early game doesn't need to be crazy powerful like it already is.