we've done it with 3. A cleric, a cabi and a pali. We've done it with a battle group. Its the easiest purple I've ever seen. Grab a cleric with FOP and you would be good I'd think.
It's usually red-grape to 50. Since you're already 50 the only reason to do this quest is to get the necklace.
With your theurgist, I'd suggest bringing a cleric and a paladin along. You should be able to do it with just those 3. Especially if your cleric has FOP. Just summon earth pets til you are out of power, have the cleric aoe buff all the pets, and let the paladin take care of the rest... rinse and repeat.