Fenrir is a werewolf capable of a sort of rational thought while transformed by being mostly insane in either form. His mate is apparently the same sort. He appears to be loyal to Roddy Lestrange. He seems to disappear in between raids, so I'm guessing he is staying at one of the DE facilities that is under a Fidelius charm. From what I've seen of him, when transformed he is no faster or stronger than any other werewolf. Untraansformed he seems to still be a fairly accomplished fighter.
I've found burning, beheading, or a half dozen silver bullets tend to do the job. Let me know if you need any extra muscle for the rescue party. I've taken care of werewolves in the past, and would consider eliminating Fenrir and as much of his pack as possible a matter of public service.
Unfortunately, I don't currently have a tracer on Dora Tonks, and am unable to locater her by the usual means. I'm guessing she is inside a Fidelius somewhere as well.