Everyone is talking about Ryze this, Tristana that...

But what about Katarina? The most overpowered all-assassin in game? She just became fotm again, and AGAIN she is wrecking everything, if she has the slightest lead over the game, she can do 1x3 as much as she wants. On team fights you can't use everything on her, because the carry will kill everyone, and because she is already innate tankish, so you HAVE to use everything to kill her.

The only way to beat a team with Katarina is if the team's overall skill is better than the enemy's, and while winning, not overextending, not even to take the turret.