Congratulations man, you finally beat it, guess I have to try a little harder to beat It myself. Also I would like to see that last attack, If you uploaded The vídeo, can you send me The link?
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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.