An occasional one hit kill isn't a big deal, though in D2:ED that was generally confined to higher level bosses using exploding arrows, etc, or possibly similarly leveled bosses getting a lucky shot in. You definitely shouldn't die if your creature gets hit, though, or with near misses.
Try emailing support@larian.com; if it doesn't turn out to just be a glitch with your install/system, I'd think they might want a couple saves to help track down the problem.
[PC] Support FAQ - The Dragon Knight Saga
