it's always possible that the trainer is causing your problems... try it without the trainer (from a save BEFORE you get to the creature) and see if that helps-
try to shoot it from a fresh load without the trainer and see if it dies-
probably it's more likely that there is a 'trick' to it or something-
best,
Cal
Ok found out its the easy kills that make them not take damage. You turn that off and it works fine.
TokenBad