RE: What gun would you use ? ATTACK!!!!
You should be so lucky that the beast would telegraph its attack route.
That 12 Ga. laoded with Dixie Slugs would be hard to beat. Hardened 730 grain slug@ 1400 fps. The Tri-Ball, three .570 diameter balls at once. Deadly to 60 yards or so. This is the game that the dixie slugs were made to play. Unlike softer slugs, these will penetrate large heavy skinned, heavy bonedanimals. Best of all the average hunter can afford this protrction. I have carried them for Bear hunting but have been lucky enough to never have needed to test them in this manner.