It's called Asrielovainia, the reason why I chose this song because it's in between Good and Bad. It's pretty simple because it's easiest to get neutral route in Undertale, so you have to fight her.

Fun fact: Dreamscaper's eyes can change colors, but only her right eye can be gold, and her left blue. Whenever in a battle she keeps her left eye closed, unless she wants it to get REALLY chaotic! When both eyes are open you better say your prayers because it's just a whole new level of crazy! I think she should open her left eye around the "Your Best Nightmare" part. It will fit in nicely there.