one of the toughest solo quests in the game is the hunter epic bow quest. This requires you to solo kill a couple of elite demons through out the game world. Normally you cannot me helped in any way shape or form.
Another person that touches the demon, or heals or buffs you during the fight causes the demon to deagro and despawn and you fail the quest and then must wait 3-4 hours for the demon to spawn again. Even if you get no interaction from other players, killing him is really tough as he has alot of hps and if he hits you, generally you get turned into a demon and die within seconds.
Well, there is a way around it if you have another hunter who has completed the quest and already has his epic bow. The other hunter can initiate the fight with the demon and since he has the epic bow, kill the demon much easier then you can. Just make sure you don't help the other hunter kill him. Only one hunter can engage him at a time.
Before you start the fight, make sure loot is set to FFA then loot the demons head when he is dead. This makes it 10 times easier to do the quest. I'm sure at some point blizzard will fix this so do it now while you still can.