If you are going to hunt with a shotgun, the best advice I can give you is know your guns limitations, and don't ever shoot at anything that you aren't sure that you can hit where you aim.
Test you gun by target practicing with it at different ranges. Each shotgun shoots differently. I can't stress this enough.
A 12 guage will bring down a grizzly with slugs, but they lose there shocking power the futher out the slug travels.
My advice is only shoot slugs no buckshot, and only shoot at an animal that is 50 yards or closer.
If you follow these rules you will bring home the venison.
Ps. Check out my blog below. Whitetail Hunters Challenge