[thejeff]

Banishment is unlikely to kill undead pretenders with a single strike. It does MR resisted hp damage. On a lucky roll it could kill a low hp pretender outside of it's dominion, but I wouldn't rely on it for killing any undead thugs or SCs.
It's great for blowing away hordes of undead/demonic chaff, especially if you've got high level priests. Powerful single undead require other measures.