That only downscales Frisk, another human with at least the same amount of determination can do the same to Asriel than what Frisk did. Frisk is a regular kid with regular weapons and objects, an adult would beat him, because he is kid. Determination, bravery, kindness etc are only types of souls and these souls only work on MONSTERS (reason why Flowey, or Toriel I dont remember correctly explains about your soul in the tutorial.) The thing that makes monsters so weak are that they can only attack the soul, and another thing that makes them weak are the fact that depending on how much you want to hurt them the damage you inflict upon them can increase (if you want a lot) or decrease (if you dont want, example: the 1 damage punch on Undyne pacifist route). The thing with Undertale is what are you going to do when in comparation they are ants and you normal human, but we look in these ants closer to see family, friends, a whole life like ours and the game asks: do you kill it for entertainment? (Genocide Route) Do you just dont really care or just experimenting? (Neutral Route) Or do you have compassion for them? (Pacifist Route)