Yes! Finally managed to beat impossible mode and recorded it. The final section of the last attack is rough. Took me about a month to beat it and I only reached the last attack four times total. Even then, I barely got passed it with one hit left. Great game.
Viewing post in Undyne the Undying fight remake comments
Here is the link.
Undyne the Undying Fight Remake (Impossible mode) by RG00 - YouTube
The final attack starts at 16:55. It seems to have some variation so you will probably see a different one. The only one that seemed consistent was the sea of spears at 17:38.