Halp me

I am trying to finish Help From The Void (Disbeleif papyrus + last breath sans) but i for THE LovE OF GOD CANNOT BEAT PHASE 2 (after undyne appears) LIKE BRO, DO YOU HAVE TO MAKE THE ATTACKS THAT FAST???

*inhale* ok, im kinda raging, but i do need help since i ALWAYS die right after undynes turn. oh and did i mention undyne gives no invurnerability frames and also does karma damage?

seriously though, WHY IS IT SO HARD??