.20 gauge? Since I don't use a .20 ga for predators, here is an educated guess... I'd seek out a "denser than lead" Hevi-Shot type high velocity waterfowl type load (NOT STEEL SHOT) with a shot size of #2, B, or BB and shoot it from a tight choke. Pattern on paper to determine point of aim and pattern density. Limit distance to less than 40 yards and I think you will do alright.