[Goronvir]

Thanks, at least I know to keep trying!
I did manage to kill it quickly the first time, much to my surprise.
Wand of magic missile seems to upset it, as do blind, slow and greater malaison - but even the sword of Balduran has little or no effect, which is why I thought there was a deeper problem.
I don't think my party is magical enough to clobber it straight away other than by sheer luck.
I would not have got even this far without help - thanks again!