Victor will be your final boss fight in The Darkness 2. He will do a few different things, but like the other bosses he is pretty simple.
Victor will send his goons at you from time to time and disappear. Kill all of them and then look for the purple smoke to determine where Victor will appear and reappear. Using the swarm skill is a good idea to save for Victor. He will be stuck in one spot and you can just unload your clips on him. You should have him beat after a few waves of his men. Just keep shooting him down and it will be over before you know it.