Hi, and thanks! I'm glad you enjoyed the battle.
Also, I already made a post about this mod in the subreddit, although I think I'll do another one when the v2.0 will finally be finished.
And don't worry, it's not that you're bad at the green soul waves, those waves are actually a bit hard, but I can't really bring myself on making them easier as it's Undyne the Undying we're talking about here XD. Although maybe I'll do a normal Undyne battle after the v2.0 (still not sure about that though, mostly because I would need to remake a lot of sprites, which is a thing I'm not really good at).