The best, easiest, and consistent place I've found is the Ultra Knight in the Cabal Ship on the Dreadnaught. You have to jump up and over the door in the middle of the ship to get to him. It's just past the big Generator looking things if you come in from the mausoleum side or just before if from the landing side. Sometimes you have to spend the time to kill all the adds before you jump over the door. Once inside there are a few more before you reach the Ultra Knight. He's easy to kill. Very easy to solo too. After you kill him go to orbit and repeat. If you just come back out he won't spawn again, or if he does it will take forever.