hello,
I'm stuck at the companions quest line. My quest is to kill the mages in rannveig's fast. My problem is that I already killed everyone at that place before I recieved this quest. And if I go to rannveig's fast I see my target laying on the ground dead, but it doesn't accomplish my quest because I didn't kill it in the quest, but i have already killed it before. Is there a way to respawn him or to solve this because I want to become a werewolf. BTW I'm playing on the ps3.

Unfortunately, it seems that the only way out of it is going back to a save before you received this quest, walking up to Companion leaders again and hoping they will give you a different radiant quest. The area at Rannveig's Fast is non-respawnable, so you can't wait 10 or so days and visit the location again.
You could also try resurrecting the boss at that location as a zombie, but I have no Idea if this can help.