I feel like this song doesn't get all the love it deserves, Megalovania is great (And so is the cover of it I did lol) but this song, in my opinion, is better.
Also, I noticed something interesting.
I don't think the time change was just a style choice from Toby, I think it actually has something to do with the story.
As the battle goes on, Undyne begins to melt, and get more desperate, knowing she is running out of time. In the later half of the song, a beat is cut of off each measure, resulting in the song playing for less time than it would've if it stayed 4/4. Coincidence? I think not.