[Farling]

Try the technology guide, that's where all the "high tech" complicated that breaks the high-fantasy approach resides.

P.S. the sack over the head still doesn't render him "helpless", since isn't any of "paralyzed, held, bound, sleeping, unconscious, or otherwise completely at an opponent’s mercy" - he's just got a sack over his head making him blind.

Now if one of you hit him with a paralysis effect, then the other could get in that coup de gras...