What happens is that it targets creatures with more powerful skills first, which, in this case, is the deathstalker, not mattering whether it will have an effect. being an automatic skill, any AI would PU it over anything that doesn't have a skill, or has a skill it can't use. it doesn't matter that the deathstalker can't USE it's skill due to it's attack, because it still uses it every time - without any effect.
(That's what i see, at least.)