Grizzlies can certainly climb trees. They don't do it often, and they don't go far, but they do climb. They are crabby, territorial, and completely unpredictable.
Unless you can get a heart shot, you're unlikely to drop a grizzly. Head shots are nearly worthless unless you can hit the sweet spot behind the eye and below the ear. Straight-on shots have been known to graze right off their skulls. Even a lethal shot to the body can take several minutes to hours to be effective. Then you just have an injured, even more pissed off bear.
Avoidance is the best option. Bells are good, so are smaller dogs; if they're quick, they can harass an angry bear away.