Soul mode red is the default soul mode. It allows you to move along the fighting box with the arrow keys without any restraint. Undyne will turn your soul to the green mode with a spear.
After six attacks, she will turn your soul to the red soul mode again. Take this advantage to flee. When you escape, run as fast and far away from Undyne as you can.
Undyne will not move during this call. After that, keep running until you reach Hotland and then you will have successfully spared Undyne.
