Sentinels can also solo champions, at least the ones from the Belsavis heroic daily quests ("A Lesson is Learned" and the stasis generators). Granted, your gear has to be pretty good, Doc's gear has to be pretty good, and you have to be watchman specced. Even then, you have to play well and use all the right abilities and cooldowns. If you do all this, the champions are actually somewhat 'easy.' I tried them as combat as well and couldn't kill them, though I came close - the healing (and perhaps extra damage and defense) from watchman is too good.