I never win on Kassadin unless my team was ahead
I don't get how anyone wins on him tbh or why ever pick him except for fun? You can use Kat as a cleanup and mobile champion too while having better teamfights and more waveclear. He's SOOOO weak early. He gets bullied by literally everyone... even a Ziggs was shitting on me. Kass has basically no waveclear so his turret tends to fall early and if they group mid, you're completely screwed because unlike a Viktor or Anivia, he just watches his turret fall helplessly. His damage isn't even that great. It's basically Ahri level - which isn't terrible but Ahri at least has poke, hard CC, some waveclear, is ranged and has similar mobility but just more situationally.
His level 16 mobility is pretty nice but the dash is short so he only feels decent not amazing.