Bloat King Boss Fight
Hit the blue orbs and not the slug. The giant blue orbs.
The more you hit the orb, the quicker it becomes a weak point that takes Red Damage (a crit zone) from almost every weapon.
The slug itself (the thing the orbs go into) should rarely be your main focus of damage.
Also, you have to i-frame the quick beam or drop down into the water.
The first is preferable due to the infinitely spawning cthulu fetus babies in the water below and those things will completely annihilate you because they literally just spawn beneath you constantly.