How can i finnish this stupid quest?
I gave the archer arrows.
But the fanatics still manage to kill everybody else?
What should i do?

exactly which quest are you referring to?

I'd guess Vitharn (SI) - it's the only one I recall where you have to fetch arrows for an archer. It's one of four things you need to do, so the OP needs to do the other three.

use console commands to mark it finished?

Been a while... Arrows to the archer, destroy the doll for that one guy, open the gate for that other girl, and I can't remember the last one.

I think the last one was finding a ghost dagger and giving it to one of the robed ghosts outside in the siege area, or something like that.

I don't do that crap.

I think the last one is to give either the dagger or a warla stone to the wizard ghost (he complains that if he only had more magica)

Even after giving dagger, destroying the doll, and retrieving the arrows, the fight will still be lost, you then need to go to the 'count' or whatever he is called, he will tell you he failed to protect the place to and give you his helmet so you can fight in his place. this can only be done after you have found all the previously mentioned items and used them appropriately, you can now get involved kick ghostly butt and continue on your travels.

Awesome. :thumbsup: