Honestly I am calling bull, maybe it is because I came from WoW where healers are God mode, but I really don't see healers being OP even tho I am lvl 25 (had an identity crisis! Marauder or Merc!). Seems like a L2P honestly from what u have said. The best strat is to never let a healer free cast and you said yourself that you guys will tunnel one and let 2+ others free cast. You need atleast 1 dps per healer while you focus the other down.