This champion is just unhealthy for the game. Whenever you leave it open, someone will just take it, and effortlessly make an unkillable tank out of anything. It just makes the whole game free for the Yuumi and who she is currently attached to, while making it a real nightmare for all five of their enemies to deal with them. Grievous wounds is just a joke for her, she just keeps healing and shield for ungodly amounts and keeping the whole team alive, without even requiring in most cases, to pay attention to the game at all.
Remove or rework Yuumi!