Made a little digging into Bosses HP values and damage dealt to them using runs-related weapons. Tried to interpret it to the best of my ability after series of tests. Completed It seems that damage values are the same across all difficulties, except last Morpheus fight.

https://docs.google.com/spreadsheets/d/12HkrwCc1pDUYRopdnPqwHVbVFtA-Rysv_E-Jkzqz9ac/edit?usp=sharing

P.S. If you hit Hunter's sweet spot with GL or just their torso with Magnum, no matter how much HP Hunter has, game will register it as a one-shot-kill. Example - if you tweak Hunter's HP to 50000 instead of 5000, and hit it with 1 magnum bullet, game will first assign "44000 HP left" value to its HP, and then assign 0 to it, confirming your one-shot. On your screen it will die instantly. Hence, the "INF" (infinite) in the sheet, because I dont know how else to describe it while staying as honest to the game's mechanic as possible.