I actually propose that our Imps are skeletons and zombies, with the traits of stupid and creepy. We don’t need them to be competent when it comes to combat and in fact would want them to be dumb enough to sacrifice themselves on our behalf. The creepiness will ensure that anything that be Orcs spot them will be scared of them, and won’t have any funny ideas of trying to investigate. 

As for how we get zombies, they’re more or less Darc’s slimes animated the remains of people and things we’ve killed. Kinda like what he’s doing with the dwarf who’s skin he’s wearing.