Wizard packs on nightfall. The one thing you pretty much can't solo kill without some kind of cheese. I like them and the epic Captains though because they are actually somewhat easy to beat with aggro control. Since they lock on their barrages to a target, your teammates have free reign to shoot them while one of you dances like crazy to avoid the shots. Definitely an interesting mechanic. Also actually dodging those barrages is really fun.