I practice MMA since 6 years now, boxe at the same time since 7 years. Also body-building since 4 years, 6.13 feet for 191 lbs.
I have a crazy speed, reflexes similar to my speed. I just have to wait bear to charge me, avoid him and give some good punches in his face. I won't let him down, to any errors, the bear is over. There always will be virgins from here who'll think it's impossible. Nothing is impossible with will first friends, et at a second time, you can't do anything with the weak body you have.
With a knife any men a few trained can beat a bear.
With bare hands, it's not so more complicated, it just requires technic.