crazy theory but anyways
It's in chapter three so first u have to get the shadow mantle item which is kinda hard to get (search up a yt guide cuz explaining will take too long) then equip it onto susie (it doesn't matter which character gets it but she's the best option because of her rude buster)
this will make her take way less damage from the knight
if u have the shadow mantle gaster will let you retry the knight fight after you die
Fight her but she's really hard I recommend watching up a guide
U can't spare the knight u have to deal 1500-1700 damage to win