So, I don't know if this is known or not, but just screwing around I found this neat little trick.
The start in the arena of sewers of dalaran, right when the gate opens, you can drop your spirit or warlock portal and then jump into the arena. If the damage gets rough you can port back up and heal, but be cautious because it will kick you back out when the water turns back on in the center. So just drop it again heal and jump down. As a MW, you can just nullify melee damage and some ranged completely while healing. Hope this helps and sorry again if its been posted.