In the new ring of blood event located in Twilight Highlands, there is a easy way to complete the quests. If you see the oposit faction has started the event, and is down in the ring figthing, there is every now and then a chance to start the event yourself. I did this with a couple of mates of mine.
We waited until alliance had fought their mob to somewhere around 20% health, then we started our encounter. When their boss died, we were given credits for the boss we had started, even tho he was just about to enter. Alliance however was not given credit for anything, they had to to the first boss like 5 times, until they realised we were robbing them.
tl;dr: Start ring of blood IF oposite faction is already figthing it, and their is low. When their die, your quest completes.