You mean his health and shields simply don't go down? They are tough to kill. They have high health and shields, and some move that makes their health drain slowly. Then, when you take out all their health, their still not dead, they regain about half of their health, they will usually be knocked down though. You just gotta pummel them alot. Biotic lift and throw help w/ them a good deal, or high physics force to get em on the ground.