I remember an amusing image from a dream last night. I was walking downtown and saw a giant poster of an upcoming match between a big steroidal-wrestler-type fellow armed with a chainsaw and an enormous grizzly bear, approaching Kodiak bear dimensions.
So, who do you think would win this altercation?
The guy with the chainsaw could get lucky, but the odds favor the bear. Or to put it another way, no one actually hunts with a chainsaw because Hollywood badassery aside, they don’t really make good weapons.
What’s the context, and how pissed off is the bear?
There are basically two ways the human can win: He can either scare off the bear through use of the loud, stinky thing, or he can use it to alter the environment to his advantage.
But if it actually comes down to a direct fight, claw vs. saw, well, a chainsaw isn’t nearly as effective as a weapon as you’ve been led to believe by movies and video games. The bear gets injured, the human gets eaten.
Hitting the bear on the nose with the running chain might turn him around in a hurry. If it doesn’t, you die. Probably quickly because you’ll be sliced open by your own running chainsaw as the bear continues the attack, and he’ll be really, really, really mad.
One slim possibility for survival is if he rears up and you manage to hobble him with the saw. You might be able to out run him them.
Best bet, hand the chain saw to your buddy, and run.
Bear.
Here’s how this would go. Man swings chainsaw at bear, it starts to bite, does some superficial damage, and then the bear takes his head off with one swipe.
Bears are freakin tough as hell. Never bet against the bear.
I dunno; sometime Señor, the bull wins.
The lion wins. The lion always wins.
Actually, a bear would kill a lion in a fight. Lions have strong jaws & front legs, but lack toughness. Whereas a bear has strong everything, and is tough as hell. The lion might get one good bit, but the bear would kill it right afterwards.
Lion always wins. Your arms are too short to box with lions.
Don’t make me post my list of “lion wins” citations I have developed over the years, because I’ll do it… and you’ll be sorry!
Depends on the type and size of bear. I think a lion would beat any black bear that weighs less than the lion (Siberian tigers routinely prey on black bears), but a lion would be no match for an adult brown bear.
Bear wins 9 times out of 10, if not more, at least if the bear is strongly motivated to kill his adversary. Even a very large strong guy is only going to win by a lucky hit on the bear’s face or hind leg. On the other hand, if the bear gets in even one good hit the fight is over instantly.
Small bears yeah. but I was going with the OP’s grizzly.
If wielded optimally, the chainsaw could inflict a potentially lethal injury on the bear - but the bear would then knock the crap out of the guy and kill him before bleeding out.
So both would die, but the chainsaw operator would die first.
The bear has a longer arm span and would knock the chainsaw out of the man’s hand. Then he’s in a shitload of trouble because he’s now facing a bear with a chainsaw.
Who wins?
The next-of-kin. Gets a life insurance payout and a cool bearskin rug! 
Chainsaw-guy’s best bet: wait until hibernation season, then fell a bunch of trees over the den exit.
Move south before spring.
Second-best strategy:
“If you don’t eat me, I’ll give you this nice chainsaw!”
Hilarious images!
Even if it’s a hairless bear? 
