Luigi and the Haunted Mansion
Ooh. Spooky. Head towards the mansion, watching out for pumpkin-wearing Goombas (you need to spin them before you can kill them) and the rolling Chomp. Inside, hit the switch to turn the light on. The Boo ghosts only advance when you have your back to them, so trick them into the light to kill them. Grab the key from the second Boo and head into one of those weird gravity-warping rooms. You're looking for five launch star pieces, and they're not hard to see. Just be careful of the black holes in the floor and the blue flames. Use the launch star to swoop around to the other side of the mansion, where we first see Luigi. Poor, pathetic Luigi. Use the pull stars to grab the key, then head back inside for some fun. Get rid of the Boos, then look in the far corner for a question coin. This opens up the Boo Mario power, turning you into a ghost. You can hover and shake the Wiimote to pass through bars. Float up to the ceiling if you want to grab an easy bonus life, then go through the bars. Here's Luigi, but if you approach him as a ghost you'll only scare him. Pass through the bars on the other side, and let the swinging light turn you back into Mario again. Now trampoline over the wall and your stupid, stupid brother will give you a power star.
NOTE: Luigi now takes up residence in the Garage back at the Observatory. Talk to him and he'll seek out new stars in previously explored galaxies, which makes him a bit of an annoying smartarse but also quite essential for those chasing all 120 stars. He'll send you a picture of where he is, and you then have to figure out which of the galaxies he's in. The scamp.
A Very Spooky Sprint
This can be a deeply annoying stage, and one that boasts uncharacteristically wonky level design for a Nintendo game. It's a race against a giant Boo, with Mario forced to swing from pull stars while his opponent glides through all the obstacles. And, oh my, obstacles there are. This is a race where split seconds count, so being buffeted about by giant spare ribs and stuck behind a revolving platform can leave you grinding your teeth into dust. And then there's a tight minefield at the end, though if you approach from the right you can bypass this almost entirely. Even more annoying, losing the race loses a life so while I'd recommend a few run throughs to familiarise yourself with the course, it's going to cost you. It helps if you let go just before you reach each pull star, allowing the momentum to carry Mario past it. This usually puts you in reach of stars further away and lets you build up a rhythm. There's also a launch star tucked away just after what I can only describe as the meat chicane, which will blast you past the skeleton roundabout but unless you latch hold and keep moving the second you land the camera angle can leave you with nothing to grab. Basically, there's no easy way of doing this, so just stock up on lives, take deep relaxing breaths and keep trying.
Beware of Bouldergeist
Time to meet a new twist on an old enemy - the Bomb Boo. Head into the mansion and you'll see that, in true haunted house style, everything has moved around. Bomb Boos will spawn from the painting, and Mario can spin to grab them and twirl them around. The longer you spin, the wider their swing. You can also move, slowly, while swinging them. Use a Bomb Boo to clear the fireplace, then head to the launch star. You'll land at a side-on section with sling pods. Reach the launch star to whoosh around the other side, then collect the pull star pieces to move onwards. You'll reach a hollow planet with a bouncy centre and mines all over the surface. The trick is to bounce up to the launch star without hitting the mines. That done, you're ready to face Bouldergeist. Stock up on health or extra life from the Luma shop, then jump down. Bouldergeist takes several forms, but your method of attack remains constant. He'll throw rocks at you - grey, gold and black. All will hurt if they hit you, but gold ones turn into coins while black ones turn into Bomb Boos. Grab these, and spin them into Bouldergeist for MASSIVE DAMAGE. Keep doing this, and dodging his punches, slams and rocky eruptions, and you'll have another star for your collection.
Matter Splatter Mansion
Another secret star, that can be found by returning to a previously conquered level. For this one, head back into A Very Spooky Sprint and spin into the sparkling rock outside the mansion - it's on the right when you land, you can't miss it. It'll turn into a launch star, so use it. You'll soon see what the trick is with this special level - the floor, walls and ceiling only exist when the roving spotlights are over them. You need to find just the right speed to stay in the middle of the solid area as it roams around. It's tricky at first - especially since you have to keep turning to face those bloody Boos to keep them from sneaking up on you - but it gets easier once you reach the piano room. This is also a checkpoint so you can give a small sigh of relief. You now have to start moving upwards, but resist the urge to wall kick - the walls won't be there in time! Just keep moving, and jumping, and grabbing the keys to open doors for bonus items as you go. When you reach the top, the star is behind one of the doors.