its possible to solo but not easy. I managed to get Azmodan for this achivement but Belial kicked my ass during last phase.

From what i read you can wear gear during whole fight but cant damage any mobs (Thorn damage also cant) and then only strip at the end. But you need to hit the boss at least once while naked i suppose.

Easy way is get ppl to help while ur naked or gear your follower if solo. My follower wasnt able to kill Belial before he breaths to kill me in final phase.

i remember achivement was the only thing that kept me playing wow last time. It gives us a goal to strive for instead of just randomly killing and farming mobs u kill for a reason lol