Assuming he's got a rifled barrel, at least as far as ammo goes, the Remington Core Lokt Sabot Slug would have to get the nod. Streamlined, jacketed and bonded, and .50in instead of .72in, it's not so much a shotgun slug as a rifle bullet.
It is a .50 caliber, bonded, jacketed, semi-spitzer style bullet, 385 grains, that exits the muzzle at 1900fps and 3086ft-lb - similar to a .45-70 round. It shoots much flatter and penetrates better than conventional 12 gauge slugs, with only 2.5 inch drop at 150 yards. At 100 yards the bullet is traveling at 1648fps and has 2325 ft-lbs of energy remaining.
I couldn't find the article just now, but when it first came out, Remington published a little article about it being used on a brown bear hunt in Ak. It worked.
Remington claims sub-2-inch groups at 100 yards...
See:
http://www.remington.com/products/am...nded_sabot.asp