Here is the full list of all 8 Strangest.io's My Megamix '21 achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Awarded for opening Purgatory!
Awarded for opening Kero Kero Panic!
Awarded for opening Windows Defender!
Awarded for opening MoneyCalulator!
Awarded for opening Arson!
Awarded for opening Hallucination!
Awarded for opening Nukeheck!
Awarded for opening the Supersensory Megamix 2021 launcher!