Cheating, Bug, or is the champ Too Strong? (Kai'sa)

I just got off the most ridiculous loss since I started playing in season 3, and I want answers.

We were ahead in every aspect (11k team gold advantage at 28 minutes in) and even had several kills on Kai'sa. However, at about 20 minutes in, she magically starts winning 1v3 fights solo. Furthermore, after another 5 minutes, she's going 1v5 and forcing us to retreat. The 11k gold advantage disappeared in 2 minutes... because of her (we were slaughtering the others).

Then she's healing for as much damage as she deals, which is apparently more than 3 damage oriented champs at once; cc is not working on her as she somehow cancels everything in less than a second and keeps moving around to catch and kill us; she didn't seem to have a cooldown, either.

So I've come to 3 possible conclusions, since we were well ahead in both kills overall and gold until about 28 minutes in: Either that player was cheating (it's actually what I think was going on), there's a bug to exploit, or Kai'sa needs a massive nerf.

Because it makes no sense for ONE champ to win a game their team was losing pretty badly.

For now, as I said, I suspect cheating. I reported them, and at least one of my teammates did, too. The Kai'sa player did admit to having their main account banned, but left without saying why. Still, in the case it's a bug or just broken champ/rune synergy design, Kai'sa is getting my ban in champ select - I don't care if I'm the support.