I don't know what was that thing you cooked for Guardian but oh my god I still don't know how was I able to 100% it. Anyways, leaving Megalovania for the last one, I've 100% all Undetale + Deltarune (up to chp4) songs available! 
Viewing post in Deltarune: Lightners Live Plus comments
I went on vacation till the end of the month the next day I finished 100% on all songs and I didn't see the reply lol
I see you've alredy beaten Megalovania (congrats!), and I agree it's kind of easy, I only missed 1 or 2 notes the first time I did it but I wanted to leave it until the end haha, but yeah Guardian it's the one I had more problems with, I don't think the sections are that difficult but the lenght and how fast you switch from section to section is what makes it that hard (also most sections being the same notes but the sides are inverted and my brain can't process it lmao)
Which ones are the songs you have left? I kinda want to see if you're having problems with the same songs I did hahaha