U need to talk to chara and tell her about muffet after that die in the fight of muffet and chara gonna help you (u gonna have an item spam it and u gonna dicrease all damage > 25% > 50% > 70% > 90%(max))
Did u spam her item ? U can have 90% of reduce damage and u take only one damage
Go talk to chara tell her about muffet and die in the fight of muffet and u will see