Did this with some friends of mine. The healers were immune on all tries but the first. But after a few tries we started with a cc on one healer then started dps on the boss, after a while he starts yelling something then you are able to damage one of the healers. Killed the healer, continued dpsing the boss. cc broke on the other healer which was immune until we got the boss further down at a point which he started yelling again, and then we could kill the second healer. The healers doesnt seem to do all that much while they are immune really, so we managed to get him down with this tac.