I encountered this same problem with some of them refusing to die &, unfortunately, found that reloading was the only cure. I didn't try any different tactics (since I knew I could... um, *should*... be able to kill them with my original tactics) but had different results.
So, as annoying as it may be, try reloading but go ahead & use the same tactics that got you there in the first place - sometimes they work, sometimes your enemies are invulnerable.
Still enjoy Ascension EndGame much more than the original, even with the chance of a wasted effort.
- Imrahil