Near the end of the 1st Act, Brutus is found just before entering the Prison Gate. You'll have to make your way through both the lower and upper level of the prison.
He will be the last encounter before leaving the area.
At half health, he will summon skeletons to his aid.
Brutus is resistant to chaos damage (I think it's called chaos).
Side Note: I've killed him twice already, once in my single player playthough and again in a friend's world as the Ranger. Have yet to receive the achievement...