There is no piece of gear worth vying for THIS HARD at lower levels that you should run a dungeon more than a few times to attempt to get the drop. If you still do not get the drop after a few runs you're probably a level (or two) higher than you were and have new gear available to you.

The solution to your example of being a level 30 tank with a low level shield is already available. You go buy one. From crafters. On the Market Board.