I'm doing the Trouble in Skyrim quest for the companions and this bug has appeared:

I've killed the bandit leader previously when I looted Valtheim Towers. So now I am left to kill an enemy who is already dead.

http://cloud.steampowered.com/ugc/613842483376568893/BD83F500F0E7FFE5ACBECD49ABCFD1E946D9F70F/

Is there anything I can do? How long do enemies take to respawn?