How do I spare Undyne?

She keeps attacking me, and no matter how many times I either try to use any ACT option, she doesn't permit me to SPARE her. Is there any peaceful way out of this?

Solution 1:

To spare Undyne, you need to survive her waves of attacks until she changes your heart back to red and throw a spear at you.

Run during this turn.

When you get on the overworld map, Undyne will be stunned for a moment. Use this to run upwards. She will catch up with you and will attack you again. Keep running when your heart is turned to red again. You need to do this three times in total.

After three waves, Undyne will no longer turn your heart green. At this moment on just run all the way to Hotland. Around halfway on the screen that displays "Welcome to Hotland", Papyrus will call you (if you spared him), giving you a moment to relax. Undyne will not move during the phone call.

On the first screen of Hotland, you have a few additional seconds because she will stop for a moment and get angry at Sans.

She will collapse on the next screen, because her armor overheats. If you want to befriend her later on you need to give her water from the nearby water cooler.