How to Use Saint Mirror
Saint Mirror can eliminate entire opposing teams in some situations without you having to land a single strike. This is especially useful against ranged opponents which often have limited HP, but it's also dangerous to high-damage melee units. For instance, Monk's Peace Maker reflects for equally massive damage back at it.
Playing against Saint Mirror is tricky, especially if your deck relies on ranged units. If you're not built up to fight it, pulling away from its location allows time for your opponent to come to you instead. Beyond that, you MUST employ invulnerability or resurrection to withstand the reflected damage and keep your other units safe. Witch's skeleton armies are also a particularly good countermeasure, as Witch itself does not take damage reflected back at her sacrificial skeletons and their damage adds up as they swarm.