[Darkman]

I think Mustard Jellies have to be hit with +1 weapons or +1 ammunition. I would have your mage cast Mirror Image and run in first attracting their attention while the rest of your party moves in to do the real damage. Make sure your party has some sort of magical aiding going on, like a Cleric's Chant spell, and be ready to cast Dispell Magic and Slow Poison if someone gets poisoned and slowed.