Super Mario 64 DS/Chief Chilly Challenge

Once you have rescued Luigi and have gained access to the second floor of the castle, turn right, and you should see a door with a star symbol on it. Open the door and you'll get to a room with a mirror. As Luigi, there will be Power Flowers here. Jump up and grab one of them, then run through the mirror. Manoeuvre your way to jump into the picture frame that has Wario's image on it. Once you're in, go down the slide, and the wind will blow you up. You'll then land on a platform. Jump over all the tilting platforms, then jump onto the platform that moves back and forward. You'll land on the next part. Turn right and freefall your way into the lake below. Jump over the next few parts, then find your way over the moving path that goes up, down and all around. Once you're on the next part, there are some elevating ice blocks, get your way all the way to the top. You should then see a big hole. Jump down there.

And there you go, a big frozen Bully. Now, what you have to do is punch or ground-pound Chief Chilly until he falls off the edge. He'll come back on and bits of the platform will diminish. Keep doing that until you're left with not much, and then he'll eventually fall off.

Note: Once you've won, you can get an extra hidden star by going through the other door in the mirrored room.

8 Red Coins

To get all the 8 red coins, you will need to have unlocked the ? Blocks to do this.

The first coin is located on the moving bridge right after the tilting platforms.
The second coin is quite tricky to get. After the bridge, turn right and jump off the ice, where the wind will carry you upwards. The coin is in the middle of the square of coins, so you have to aim correctly. If you fall down to the lower ice, go back and try again.
Fall down to the lower platform and jump in the freezing cold water, where the third coin is. Don't stay in too long, it takes your health away.
The fourth coin is right in the middle of the next platform with all the bullies.
Jump onto the moving platforms. The second moving platform will line up right below the next red coin, so stay in the middle of it and wait. The same thing will happen with the fourth moving platform.
Move off the platforms and head around the mountain, where there is a Mario cap. Climb to near the top, and get the Power Flower in the ? Block. Float quickly over to the lone platform, and the seventh coin is right in the centre.
Float back to the mainland and get hit by a snowman to go back to your normal self. Get another Power Flower to turn invisible and run into the cage, where the final red coin is.

Grab another Power Flower if you need it to get out, and head around just a little bit further than where Mario's cap is. The star is there.

Secret Star

You need to have unlocked Wario and the ? Blocks before this. Once you have done that, grab Wario's cap just after the tilting platforms and head over the moving bridge. Go to the left, grab the Power Flower to become metal, and run quickly but carefully over the bridge. Normally you would be blown off, but as you're metal, you aren't. Break the Black Brick Block at the end of the path and collect the next star. You can get back with another Power Flower.