I don’t know if this has been confirmed elsewhere or if it will help anyone, but I beat Pure Faction Mirrored Halls with very minimal effort. Definitely on my list of factions to recommend to players just looking to max one with minimal investment—all this faction takes is time and patience in my experience, but victory is assured if you don’t quit the match.
The basic strategy is:
Copycat
Mirror Queen
Copycat
Mirror Queen
Maintain that order with your own summons, sacrifice anyone you see is about to die so that you don’t have 3 injured troops at once (thus making you vulnerable to skull cascades), and cast your Mirror Queens against the opponent’s reflect-makers. If you’re the only one who can make reflects, you simply win by attrition.
Be sure to always try to kill their strongest copycats first, so what the remaining troops summon are ever weaker as time goes on. Be sure to keep this weakening in mind for yourself, too 
No video taken because I was honestly surprised to win so easily. Plus, the video would have been like an hour long 