Random DPS drops in Massacre

First time running massacre mode.  Anyone know why DPS randomly drops to 10% in Massacre.  It drops and goes back up and will drop again randomly. 

Pretty sure its the Siren

"*NEW* The Siren:  She enchants enemies around her, making them take permanent unstrippable reduced elemental damage based on her elemental affinity. She also flees, and Defenders who defeat her before she escapes are handsomely rewarded."

Even though it says 'elemental' my normal DPS just drops to 10%.  I always prioritize and try to kill them asap, before they get their buff out. 

It's definitely the Siren. Physical damage is alos one of the 'elements' it can have. Real nuisance since all high DPS comes from Physical.

its a bug i assume the siren does have that but theres times when the siren isnt there and they have it the physical res buff and its so nasty on ogres its not even fair tbh or sharks makes them so hard to kill

The Siren can cast a spell that puts a permanent resistance on mobs it affects. Seems to cast it real fast too.  I agree, it's ridiculous. When I was playing on Massacre, my final wave took forever because I had to kill multiple ogres with these buffs. It's not like it was even an added challenge. My walls held up the whole time. Just a time waster.