I have a few friends in Alaska who do the 12-gauge-with-slugs. Some prefer a rifle. I hear .30-06 with soft points will probably do the job (after all, you're taking a frontal chest shot, the bullet is going to have a chance at every organ they've got). But what you really want is a .45-70 with big (400 grain) flat or hollow point. Which is basically doing the same thing as the slug, going for maximum impact and wound diameter at the expense of penetration and range.
Either way, long guns are definitely the best route against anything bigger, stronger, and faster than you are.