Eh, backtrack and buy a ******** of food. Memorise his attacks and get used to dodging him. He'll take a few trys, but hes one of those guys that when you beat him your more likely to beat him more often.
It's a bullet hell game essentially. If you're familiar with the genre, it typically ends up being relatively complex and often times hard patterns, but theoretically possible to never get hit even once. All things said, Undertale is a pretty light-weight bullet hell compared to the likes of the Cave or Touhou series, but also incorporates some rather unorthodox patterns that you wouldn't really see in either of the aforementioned series.
>using ballet shoes to kill undyne before she gets to the "oh god there's ******* spears everywhere" part
I guess it counts, but... I really wanted to see no-hit of that part...
* Sweet Asriel...
* That no hit sans w-was insane...
* unfortunantly Technically It was ruined at the end since he was forced to take damage...
* Kinda sad putting so much effort only to be forced to take damage...
to fight him you have to kill everything, there isn't any variation to his fight. I was thrown for a loop while doing my first run because i knew he was a boss but not that he was locked.