Start off by mashing Toriel's text immediately. Then simply follow her as quickly as possible around the mountain. You will see a rock in the room after. Interact with it then mash until you get the battle. Your time here should be optimally 1:00. In the battle, fight and strike each blow as close as you can to the center whilst not quite in the center (a.k.a. the bit just before). The most damage you can do is, I believe, 14. Be ready to fight immediately after the unskippable dialogue. At least, you will need to attack a total of 72 times. As long as you get the timing near-perfect, then everything should go your way.

You steal my world record, though, I kill you.

Kappa