Hamlet HD - Put your mind together and grab someone else's mind, completing Sierra 117 with the Blind skull in your possession at the end of the level. It's like you're playing Oddball versus the Covenant. Available on all difficulties!

Hardpoint - Complete Midnight without dying.. and by the way, no heavy weapons. Avoid the temptation of weapon caches. The best way to do it is on Legendary, but the other difficulties are available for less points.