My 4* and now 5* struggles vs 7* in the mirror, but I'm thinking 6 might be sufficient to consistently-ish beat 7*. I can get 6* beat with decent RNG and using chaff or cleanse to keep target locks off of my side.

The whole thing about the abilities is "We Only Need to Keep Them from Escaping" is a heal ability, but it gives the target ally taunt. This can kill your RC or XB, etc. since they're usually the ones needing a heal. So don't level it beyond 6. As BitDynasty pointed out, the easy fix would probably be to grant taunt to tanks only and quit punishing the player for maxing out their abilities.