Just found this by accident when wiping on Festergut hc. A hunter that knows wtf he's doing can pull (or taunt) the boss, and pop deterrence before he hits. Then, with some luck you can manage to do this 3 times (usually 2), which means a +30% (and otherwise +20%) damage bonus for 2 minutes. After 1.5minutes, repeat this, and after that another time. this allowed you to get +90% bonus damage, which really helps. Remember, this is a bit risky so only use it if you know how to play.
This might work for other classes as well, but I'm not sure.
-Aldun