If it were me, and I’m glad it’s not, I’d opt for a Barrett M107A1 semi-auto in .50 BMG with the 20-inch, not the 29-inch, barrel. I would also get Barrett’s 50 BMG M33 Ball ammo, which is 661 grains, FMJ, 2500 fps out of a 20-inch barrel. It packs 8,127 foot-pounds of energy at 100 yards. I think anything that gets smacked in the skull with 4 tons of impact is going to be in serious trouble. And if you haven’t convinced the creature that you’re serious with the first shot, you have nine more rapid shots.