The two skills you can get for beating Mirror Frisk are Shield, which protects you from the first hit you take in every battle, and Rage, which powers up your attacks the more you get hit. Each is designed to be beneficial for their corresponding route (Shield for Pacifist, Rage for Genocide), but no matter which one you choose, it will be useless in the Sans fight. Because Sans' attacks hit you once per frame, Shield does basically nothing, and since he only has 1HP and dodges your attacks, Rage doesn't do anything either. Both skills make the game easier, including the Undyne the Undying fight, but do nothing against the most pragmatic and intelligent monster in the Underground. Sans was designed to be the most difficult fight in the original game, and the mod makes sure that it stays that way.
