How do you beat the Grandmaster Galaxy in SMG2?

If you can't beat the Boomerang Bros. straight-up, there are a few tricks you can try:

• Have Player 2 poke them a bit. Even having just one of them out of sync with the other two significantly decreases the number of stuff you have to dodge at once.
• Duck. Boomerangs can't hit you when you're crouching, so you can easily crawl up to one guy and smack him.

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.