I believe it took me less lives to beat bouldergeist in daredevil mode (about 3) than in the normal mode, don't ask me why or how

I just took down the S.O.B.

After concentrating on other stars and not thinking about Bouldergeist after a while, I finally beat 'im, on my 3rd life this time around. My strategy was loosely based on some of your suggestions, i stayed close to the body and kept running. Instead of going to retrieve the ghosts when they came out, though, i kept running and let the amount of ghosts floating around build up, hoping that one of them would be behind him when he stops to punch or ground slam with his hands. After he had realeased two rounds of ghosts and rocks, (they're werea total 4 ghosts on the playing field), i snuck behind him as he was punching, actually grabbed 3 of the ghosts at once, and broke through his freakin armor shell in one swing. Then i just cleaned up and busted his obnoxiously dangling uvula, and needless to say it was extremely satisfying.

You know which one was hard? "Volcano Galaxy" with just one energy unit. Heck, I died at least 20 times on that one.

Also, anyone knows how to beat the hidden Star on the... that galaxy that has cannons and such, on the Garden... where you must clean crap using bombs within a minute or so?

Trash Cleaning Spoiler:  Look for coins embedded in the ground.  They indicate where to toss bombs.

^w00t thx I'll try that

Luigi's purple coins was by far the hardest, though was much easier after I realized I didn't need to get every one.  Only took me a few tries my second time through (if you know what I mean)

I found the big Magikoopa harder than bouldergeist however (that actually got easier when I found you could SPIN and pick up the shells, and I didn't find that out until the second time through *facepalm*)

The cosmic comit levels for luigi were pretty tough,, probably harder than any other star for luigi.

For the cosmic races. Hold Z and forward on the control stick as soon as the countdown goes to 2. You get a speed boost.

Hardest star in the game IMO: blowing up the trash in the dreadnaught galaxy. Yes I know where to throw the bombs. It took me forever to beat 2 nights ago. 

Bouldergeist was pretty easy for me, hardest ones are

1 Dreadnought purple coins
2 Luigi's Purple coins
3 Daredevil comet in lava galaxy

Those 3 made me get a couple game overs.

They always seem to include a 1-up mushroom in the hardest areas, so that you can continue trying indefinitely as long as you can acquire it during each run.

Luigi's Purple Coins is definitely tough. What you need to do is just forget about how many coins you've collected. If you're looking at the coin count, you're not looking at the screen and that's going to get you killed. My advice is to look for areas where there are bunches of coins. For instance, in the corners where there are bunches which you have to spin to get, and also the long jump sections, where there are a lot of coins in a row. Use these to build up your confidence, and put yourself in a position where you're not stressing about where your next coin will come from.

The Dreadnought's Purple Coins were also tricky. The hardest part, I think, is when the platform goes vertical, because it's harder to line Mario up with the coins than when he's going across (either right side up, or upside down). My advice is to just jump on to the platform and stay where you land, because generally that lines you up just right with the coins and to spray starbits at any projectiles that head your way.

With the tash cleaning bit on the Dreadnought... you have about 2 more seconds that you need to finish the level. The problem is 3 seconds isn't enough for a bomb to go off, so you need to get every bomb to land on one of the yellow dots. Do that and you're fine. I find sometimes, if you've got trouble aiming, just run a little bit further so that you throw the bomb straight up instead of diagonally.