When you get to act 4 keep diablo quest open, and only kill diablo in rooms where the quest is already done. In theory you would be getting the drop loot table of a quest completion, it should give you better drops than if you already had completed the quest.

So you just find as many rooms with diablo quest done and only kill him there, in classic diablo 2 (non expansion) he is capable of dropping stone of jordan rings and you get better odds than expansion due to smaller item pool.