There is literally a path on the outside of the room to kite the mobs. If you keep cutting inside to the arena and out to the path the mobs will follow and get stuck slowing them down a bit. After you get used to it you'll never get hit by anything. The Tank should be able to handle just taking boss damage and it should be burned down while you are kiting mobs. The adds don't even appear until the boss is around 50% anyway so if the Tank can't survive 50% of the boss with just a regen on them they have issues.