Have you played Pokemon Puzzle League? Allow me to explain. By the end of the game, you play against MEWTWO. It's a fairly hard and challenging match under Normal settings. BUT
When you defeat MEWTWO, you're granted a Button Combination that unlocks "Very Hard" difficulty.
In "Very Hard" regular matches are challenging. By the time you reach MEWTWO... it's literally a matter of TAS speed gameplay.
Literally MEWTWO moves so fast, that your reaction must be Tool Assisted Speedrun like. I've been able to defeat MEWTWO once. After 3 days of continuous gameplay.