Act 3: Belly of the Beast - 5. The Face of Darkness
Okay, don't freak out, listen to me. This fight is so easy it should be illegal after all the build up.
Run to the left, near the ammo, and aim for the belly under that belt. Don't get too far to be past his last claw, make sure you can shoot his jaw when he rears up. Shoot his jaw when he roars and then reload to chase him. Get in the same position and repeat this tactic two more times. I know the Dark Wretches are about, but ignore them. When the boss scoots back to the wooden platform, just shoot the two blue clamps on the ground to end the fight. It is really easier than I explained so don't overthink this fight. Also, ignore the ammo as you fight.
Moving along, you need to move along. Some ammo behind you, but you don't really need to waste time.