[Taqwus]

At least in single player I'd consider using it early, and not just to plague the AI -- early on, the experience boost is pretty much a guaranteed Hall of Fame entry. Something like quickness would come in handy.

A dusk elder, 'though, does cost 20 gems... might be a bit much to risk depending on how early it was and how much death gem income one had. Other summoners could work too. And in multiplayer, what if you run into, say, a C'tis Shaman casting Curse? Curse Lasts forever, even through resurrection by priest-recall or immortality.