Instructions
Back away from the boss as soon as the fight starts. The boss will cause enormous damage if you get within reach, so you must keep him at a distance at all times.
Switch to your shotgun and attack the boss from a distance. The shotgun spread will prevent the boss from dodging and will cause damage even from far away.
Stop shooting the boss when he starts to flash red. He is invincible during this period. Use the time instead to put some more distance between yourself and your foe.
Shoot the boss's outstretched hands with your revolver when he levitates you off the ground to make him drop you again. Switch back to your shotgun when you hit the ground and continue blasting away. Repeat this sequence until the boss dies.