Use the Felhound Whistle to summon a Fel Guard Hound. Take the Fel Guard Hound for a walk and kill some Deranged Helboars. Search for the Shredder Keys in the Fel Guard Hound's "leavings." Return the Shredder Keys to Foreman Razelcraz by the mine northwest of Thrallmar.
I finally got my shredder working but I seem to have lost the keys. I keep a pack of felhounds to protect my camp. They don't do a very good job and they like to bite me a lot. They also like shiny things. I think this one felhound I have may have eaten my shredder keys. I'll pay you if you'll take my felhound on a walk. Kill some helboars and let him eat. He'll do his business. When he's done, see if you can't find the keys in his "leavings."