This would do nicely.
However in the event of the lack of technological ability, I would suggest biological warfare. Naturally, the nasty creatures are like carrion-fowl, so all one needs to do is spread some rotten, disease-ridden meat out on an old battlefield and wait for the suckers to take a lunch break. After ingesting the disease, they opt to use their sick days while your army moves in and kills 'em while they're down and out.