If you want to know how to beat heroic Karazhan Magic Mirror this video by Asmodeus takes you through step-by-step. He does a great job of explaining his strategies and how you can finish this heroic boss.
He completes this boss with Druid. Using ramp to get an advantage in mana and dropping giants. Early tants are key to stopping some of the damage, then you can start duplicating your high-value minions.
Magic mirror uses mirror entity secrets too, so you can exploit this by dropping deathlords. It’s an interesting game plan, check out the video to see how he does it.