Mirroring is pretty much the only option for defense, since that team works against almost anything, including, often, itself.
But there are teams that will beat it. I'll give you two hints:
1) KK sings so soon because his movement speed is so high.
2) At some point, KK is immune to magic damage.
That said, there shouldn't be a "beats almost anything" team in the game. If the top 100 or so teams in fight pit are all the same, there's a problem with the game.