I've been stuck on the Grandmaster Galaxy for a couple years now. I can make it to the Boomerang Bros., where the goal is nearly in sight, but I can't make it past them?
How do I do it?
If you can't beat the Boomerang Bros. straight-up, there are a few tricks you can try:
Practice, practice, pratice.
If I rememeber correctly, there are six parts. Before I tried to tackle the entire galaxy under Daredevil Run conditions, I practiced each part in normal mode until I could do them pretty consistently without dying/getting touched. That way, I had my whole route mapped out when it was time to do the Daredevil Run.
That being said, crouching when you get to the Boomerang Bros. was a revelation when I saw someone do it on a video. Crucial tactic. Also, pause the game and dry your nervous, sweaty, hands off.