Oooh...no, chainsaw? Entirely too close of a combat weapon to be effective...you'd need a 6' bar just to remain outside swatting range...
The right tool for this is definitely a 10ga shotgon loaded with slugs, OR a .50cal chaingun...of course then you need a vehicle to mount it on...go with the 10ga, make sure it's pump and holds a minimum of 8+1 rounds.
Then, if you encounter a griz, close your eyes and hope for the best...
edit: Oops...forgot this was dogforum...please, carry on with your chainsaw discussion...