Berserker requires kills with an axe. A regular axe, fire axe, or tomahawk will work for this play style. The two easiest places are King of Chinatown and Rosewood - Orphanage Halls. There are several axes in Rosewood and they are all either in or next to a glass emergency case. In King of Chinatown you can find the axe sticking out of a wooden crate on the far side of the square. It's not quite as easy to find but if you find it then the play style is easier to get.
Once you have the axe, start attacking people with it. After getting 5-10 kills, it should pop. At Rosewood, this means armed guards. In Chinatown, you can kill any of the crowd members.
