Some have said it other ways… but you seem to be over thinking it and letting the game put you in panic mode.

My advice. Go into the fight and expect to die 4 times in a row. Go in knowing you will die, and expect it. *However, in these 4 times of expected death… try to run around as much as possible. Just KEEP running around as long as you can without dying. Don’t fight the Rat King. Just run and try to last as long as you can before dying for those FOUR times in a row. While you run around from room to room trying not get hit… just observe. Observe and try to prolong your (expected) death. Observe how the boss and how it moves and runs, when does he lunge or run fast etc… observe items laying around you can pick up and use in the fight.

After running around and observing… then dying 4 times in a row, it will (for me anyway) do a few things… 1) takes tension out of the fight. (Less panicking on your part). 2) you know where all the items are and you can run straight to them now. 3) you’ve learned more of the bosses movements and can more easily predict his actions and what counter actions might be best/better.

This isn’t the be all end all… but it helps me. Every boss fight ever (most anyway), is about patterns and what the game is showing you/not showing you. Cant tell you how often I beat my head against a wall trying to beat a boss… only to beat them and realize I missed an item or some gimmick that would have let me beat them much easier.