How to beat Asgore in True Pacifist?
Solution 1:
Unfortunately, the true pacifist run is not available until you beat the game once.
Afterwards, keep in mind that the pacifist route requires you to not gain any exp, so that means you can hit monsters as long as you don't kill them. This means you hit Asgore until you are given the option to spare him. Then, the broken mercy button appears, and you can click on it.