Depends on how big it is. The bigger the harder to kill. There was a bear that was killed by rangers who had attacked wounded and killed several people. It was killed with a .458 WinMag. When they did an autopsy on it they found numerous 40 S&W, .357 Mag and .45 ACP which did nothing to it.
In its head?
I would like to see some verification of that.
There have been some myths floating about the Internet which tended to build over time, which sound similar.
There was a world record Grizzly, killed in Canada in 1953, by an Indian woman with a .22 single shot using .22 long ammunition.