Indeed, assasination spells makes retreat order a very hazardous one.
This conversation of yours reminds me of a game where I was ea TC and vfb was R'lyeh. I just kitted out my cyclops with MR items to keep his soul slays busy. None of them got through (Not a big surprise with that high MR). Too bad something got him blind a few turns before, so he was somewhat useless in close combat with his hordes of trolls, or whatever he was throwing at me.
Pic of the aforementioned cyclops here.