DPS... they are a dps race race....the longer you stay in combat the more wisps are created.
I could never defeat them with 1h+s or bow. With bow the wisps get in the way. With 1h+s you simply don't have the dps.
After they made trolls and ogres stun immune...I decided enough was enough and levelled up dual wield.
Needless to say wisps, ogres, trolls, centurions are no longer an issue as long as you keep out the red.
Personally I think making NPC/monsters immune to playstyles is indefensible in PVE. It forces you into fotm builds just to survive. Then all you see is clones everywhere. flappy..flappy.
IMHO knockbacks, stuns ect should always work on everything or there is no point have the skill in the skill line.
One thing I have noticed about the Whispmothers is if you concentrate on the original one, the adds die when she dies.
Of course the Whisps that come along for fun are another issue too.
They were the worst mob for me until started doing this - she doesn't pop the clones until she is hurt, so keep focus on the one that's damaged and blast with all you've got. If you fight the spawn wisps while you're fighting her, you'll die.