This is how it works (as far as I can tell it works on all difficulties)
- begin the Unexpected allies quest
- Find Adria and begin the boss fight
- you can un-equip all your weapons and gear as for this to work you will need to die. (this can be done before step 2)
- make sure you position the boss next to Adria (or at least somewhere where she can attack him, she does about 4k damage a hit)
- After you die she actually continues to attack the boss, eventually she will kill him even if you are not there, however the loot and gold will be there waiting for you. In fact the quest will actually complete itself without you being there.
for the purposes of testing this out maybe you could leave about 30-40k life before dying so that you can see for yourself that this does indeed work