Any sentry will drop this. However, I have to kill about 20 to get a piece of armor to drop, and then there's no guarantee it will be the shoulders. Anyway, for whatever reason, Enots and Kcor at the gate are ALOT harder than the blue sentries inside. With 20 mana lost per caster, me (level 39 Ranger), a 34 Wiz, and a 42 Cleric dropped each blue. Mr. Enots, took us out. I died, casters gated. He has a TON of hp.
Good armor, though. Hope this helps.