So the way that's set up is that anytime you got hit for more than 5 damage Mirror of Yata activates infinitely, so an enemy that deals 5 or more damage literally cannot hurt you. Also generally speaking we want status effects so Mirror of Yata would cause an effect like, 

[b]Reflective[/b]: The next time this unit takes more than 5 damage, the damage is instead dealt to the attacker.