You probably hit his enrage timer. Most bosses start to do increased damage after a certain amount of time has elapsed from the pull. I have never hit it on said boss, but i have seen them on other bosses, and the enrage is impossible to heal through for any extended amount of time.
This. Unless your companions are in full rakata gear I couldnt see you beating enrage timers. Even then it would be pretty close. Make sure your using your heroic moments and popping any legacy abilities you have access to.