Super Mario Galaxy 2 - Grandmaster Galaxy Tips
The first time you enter the level, the Choppahs will be in a very awkward position, so just die once and retry, it will be much better.
The second part is some switches, if you just keep moving, the lasers won't hit you.
The third part is still not very hard, just make sure to refresh the Cloud Flowers every time you can. Also, make sure to take 3 clouds with you, this is what makes the next section much easier.
The fourth part is where it gets harder. You can use 1 or 2 clouds here, but with enough practice it's possible to get through without spinning even once too.
At the fifth part use star bits on the paragoombas. You can also use a cloud to get past a pull star section if you want.
Hopefully you still have a cloud at the sixth part, since it makes avoiding the hammer bros. much easier. Then, at the boomerang bros, just crouch and they won't be able to hit you at all.