game: undyne the undying remake
game creator: unsure ( colorized version by @Karapaes )
pre-battle theme: but the earth refused to die, at actual speed (ingame version is slightly slower)
actual battle theme: battle against a true hero, dual remix

difficulity notable diffrences:
easy mode:

undyne never dodges
undyne never slashes with her spear
you always have 1 shield
you have 60 hp
undyne has decent defence

normal mode:
undyne occacionally dodges attacks
undyne slashes with her spear later in the fight
when you get her to around 25% hp, undyne will start using 2 shields
you have 56 hp
undyne has very high defense, to the point where most attacks will deal 3-digit damage

impossible mode:

undyne almost always dodges
undyne often slashes with her spear from the beginning, and even do so out of green attacks
you start with 2 shields
undyne has noticably low defense, to the point where attacks in the middle will often deal 2000+ damage
undyne will steal your turn if a timer in the top right runs out, with you having a shorter and shorter time to do something as undyne nears death
when undyne's hp is completely depleted, she will sweat for a bit before launching a long final attack, combining both red soul and green soul