"I personally would want the 20mm Barret" I'm assuming, for discussion's sake, that we are limited to non-destructive devices, AND that we would want to preserve as much of the meat as possible.
Believe it or not, in a discussion of this same topic in my shop, one guy SWORE that he could do it with his trusty bow and arrow. Lucky for him, he'd never had to prove it. He was a weird one, BTW.
In that case I will opt for a .50 Barret with the advanced muzzle break. A .460 weatherby or .458 win magnum or even a .45-70 might be enough to do the job with an accurate heart shot but in this encounter might will not cut it. Other options would be a .700 nitro express or a 2 bore rifle. Clearly one would need a 2nd or 3rd shot available.