I ran into these nasties in the phobos lab (Now im 100% sure i like the base better), and i was unable to kill a sinlge one. In my first encounter it was in the small cave, where i expected to melt norml pikies with my combat shotgun, but things went badly and i teleported. The next one i saw killed me. Knockback seemed to happen only rarely, and even when corner shooting one at max range (cateye max range) with a combat shotgun i didnt manage to get him beyond scratched until he was eating my face (not a native english speaker, so if thats a double-entendre please ignore). I could have used the rocket launcher (im pretty sure i will next time, IF i enter that damned place again), but is there a normal way to kill them? Specially if you run into them at close range (it was ignorance tho, expecting pikies, next time it shouldnt happen). Now, maybe it is the lack of a "pain" sound effect, but it seemed my chaingun was barely hurting the things. So... anyway, i had SoBx2 THx1 and an A-modded chaingun, and it seemed to fare poorly.
Also, this is the first nightmare something i meet in UV, the furthest i've reached was the halls of carnage, where the lava speed got me (another nasty surprise), so i must ask if i will be meeting another nightmare thingies and if they are as nasty as these. If so, and the answer is just "nuke them from orbit", i will take out the rocket launcher and do as told.