I used to have loads of trouble with my Sentinel, but with the recent content I found I was utterly destroying it with Doc. Make sure your gear us upgraded (Doc's, too) and play around with rotations. There are some seriously killer fast damaging combinations you can do.
The other option, if you can't take a few hits, is to do what others have suggested and upgrade Scourge or T7s gear and let them take the hits while you murder.