Kat can be tough to deal with in ARAM when, due to simple luck of the draw, you don't have anyone on your team that shuts her down.
Even in these situations, though, you don't have to just stand there in Kat's ult and try to take it like a boss. In fact, that's the very last thing you want to do. Bait it out of her and get away from her when she starts spinning. Use your flash if you have to. Then team up on her and erase her while her E is on cooldown because she didn't get the reset.
The problem with Kat is that beginner level bots deal with her better than most of human players I meet at my roughly 800 MMR. This doesn't call for a nerf. This calls for people to understand that they're not playing a JRPG where you basically just stand there and exchange damage with the enemy. Sometimes you really have to outmaneuver your opponent. That really ought to be obvious already.