How big is the undy? Undy's are really hit and miss with tankmates, and mostly miss at that. Mine has been in solitary confinement for the last 3+ years, but when she was smaller I tried some different fish (eels, groupers, wrasse, etc.) and she terrorized them all, even if they were 2-3x her size.
I think the best tankmates for undy's are definately other large, aggressive triggers. Bluelines, titans, clowns, big humu's can often hold their own. Just try to add a trigger a little larger than the undy and see how it goes. Make sure there is plenty of liverock, etc. for territories to be made. Undy's will take the best cave, but there needs to be some refuge for the other guys too. Let us know how it works.