IMHO, Downy Dunstan and Spitfire in Coerthas are the toughest, though are surely doable with a good group. ^^

Alux, Bardi, and the Prince of Pestilence you could probably duo by the mid R30's. As a CON I duo'd Alux with a friend once when he was in the way of our levequest, sometime in the low-mid R30's.

Good luck with the NM hunting!