Maggots is a great spell to stop raiding banelords. A 1N mage with one nature gem can route a fully kitted out Banelord. Just make sure your nature mage is in the middle of about 15PD so the BL doesnt fly over and kill your mage on turn 1.
Poison weapon doesn't need to deal even a point of damage, hitting target is enough (unless it's considered a bug and has been changed in a patch). I've killed prot30 Dai Onis to poison damage, even when they didn't get a single scratch from the poison arrows. Slow, inefficient, but sometimes all you have.