This is quite simple. When you go back up into his room from the checkpoint, instead of blasting away as usual, continue running backwards and turn the corner as if you are backtracking to the next area. Once your character starts talking about how they feel the crown is close, go kill Manglemaw....aaaand voila. Loot.
Do not destroy the door and spawn manglemaw though. Run IMMEDIATELY around the corner and get your characters speech. That is the loot trigger. If you spawn him before then, you will not get loot still.
Blizzard...is going to be so mad at us. This is why we can't have nice things.