What makes the Sans fight in Undertale so challenging?

Jcw87c2-sans-fight: Undertale Sans Fight Clone sans fight Undertale Frisk Sans Battle Handmade Diorama - Retro Gaming Cube sanskrit Hey there! This is my Underfell Sans fight! Remember, in Underfell, Sans is a pacifist boss, so sparing is necessary for the real ending

sans fight I know the Sans fight is supposed to be unfair, but this attack just isn't sightreadable at all · WHY!?!? · FIRE IN THE HOLEEEEE · ITS ALWAYS