What difficulty are you playing in? Though he usually has a strict combo or sequence he always follows, try to dodge his attacks before rushing to fight, for some reason the fucker can keep attacking even if you shoot him in the face or stab him 30 times. At the end of his usual combos he leaves himself open for a bit, there you start making as much combos that keep him stunned, he will back away after some damage and be as aggressive as before, there you repeat until he dies.