During your turn, click the button fight to attack scratch cat, and item to heal, during his turn, use the arrow keys to dodge his ball attacks, as the game goes on, the more balls appear. Scratchcat may go down lower than 0 HP and not die, this is supposed to happen, as the item button would be replaced with MERCY on this turn, allowing you to spare him ENDINGS: Genocide, you kill scratchcat...you monster Pacifist, You spare scratchcat, hurray!
Toby Fox for Undertale