He or she would turn T-Rex on you in one minute. If the brain was larger might take half a minute.
We hear stories of 3 pound chickens going for the eyes. At 300 hundred pounds your chicken would sling you around like a rat on a stick. Then the burly fowl would slam you on the ground a few times to break your bones so you would be easier to swallow or dismember. Only to have a 350 pound flock mate snatch your wretched carcass and run like the wind. The massive chicken would probably put you down and claw your abdomen to disembowel you to get to the juicy stuff. The rest of the flock would arrive and your remains will be dismembered and gulped down by the strongest members.