There was a lot of discussion around this card some time ago. Personally I tried it in an Assimilate deck and a Monster deck. In the First case I had a moderate success, in the second I kind of failed, but might retry in the future.
In general you have to keep in mind a few things with Master mirror:
- you want sacrificeable bronzes, mostly 4p
- You need a faction whose Legendaries are strong and low effort (don't require synergies)
- you don't want said legendaries
- you need to protect him somehow
I've tried it with SK sacrificial vanguard - that is the best leader to control what cards you have in your right side, but you still have little control on what's on your left and its gonna get transformed, you have to use mulligans to control that side.
But as a faction, SK has a lot of legendaries that aren't very useful in most situations, so it might not be the best faction for it. And at 6pt, i wouldnt worry about MM being removed, at least not in 1 turn.