With the 1.05 update:
By switching to Very Light and using Navigation Assistance and Enhanced Dodge with either Traversal Assistance, Text-to-Speech, Combat Audio Cues, or Traversal Audio Cues you gain temporary immunity each time Navigation Assistance or Enhanced Dodge is used. This immunity is removed when you fire a weapon.
Other things that may help:
- You can enable Slow Motion via Options → Accessibility → Combat Accessibility → Slow Motion → While Aiming or Toggle (depends on personal preference). This may assist in difficult combat encounters.
- You can also feel the boss lumbering towards you based on the vibration feedback from its footsteps. Big rumbles mean it is close. Run away to regroup.
- The boss is extremely lethal at close range. Navigation assistance (Options → Accessibility → Navigation and Traversal → Navigation Assistance) will direct you away from the boss; try to get some distance, land some shots, then keep moving.
- Using the flamethrower in short bursts and pipe bombs can cause the boss to stagger.
- Create distance by standing near a doorway if the rat king breaks through it takes some additional time allowing for one or two additional shots.
- Focus on the big enemy once it's down the other will scatter.
- If you are having trouble hearing certain cues go to Options → Audio → Volumes to turn down the Music and other volumes.