I do know of your problem, i've tried using un petrification items as well for the stone gun and I think this is why, I think at the start of the battle, any and all effects occur to your character, but they go in the order of Weapom, Shield, Hat, Armor, Accessory, so I think the gun turns you to stone before you become resistant to it, or the square just thought that would be to easy and still made you work for it, I'm not sure