Well in BT there is a pac after the "extra" Boss that for some reason does drop assasins hp down even if they arn't in combat! I have done this with 2 assassins one as tank and one as dps and both lost alot of hp at the same time.

I dont see the OP healing being as bad as people make it seem offcorse it could be better but so could alot of other things be.Imo with some what good gear u wount have any problems healing most of the HM`s or well i dont have.