I've seen quite a lot of level 25 teams using pets with the Chew ability. Pandaren Monks also pop up every now and then. They can decimate you with Blackout Kick -> Takedown if you don't switch out on the stun. Even if you do, the switch out gives them a free turn.

I've had some success with Lil' Deathwing. He can open with Elementium Bolt, buff its damage with Roll, then Tail Sweep. At the end of Tail Sweep's turn, the Bolt hits, dealing massive damage. It'll usually knock out the opposing pet, but if not, it'll stun them for a turn and more than likely proc ...