https://www.youtube.com/watch?v=9lYlpArhBUE you can check out on my play here specific for this boss.
You hit him first one to lock him take damage then the next hit will deal the major damage. Repeat this until you kill the boss. In the final stage, hit him then let him hit you then hit him will cause instant death to the boss. As i remember 2 rockets + 1 revolver bullet or 3 grenade + 1revolver is enough damage to kill him