Hell's precipice : Extra

My party and I just tried to do the extra : we went to the prophet who told us to kill 3 titan lord.
At this moment we had the indication : "Extra : kill the 3 titan lords" and, how surprising , 3 titan lords spawned ... we killed them...killed everymob on the mission map (or so it seemed) but the extra wasn't completed.
So I ask for tips, help, or anything from someone who have already done it.

Thanks by advance (and sorry for my poor english).