This page offers a guide on the hidden level Boo in a Box, which is found within Bubble Blastoff in Deep Dark Galaxy.
Swim to the Bottom of the Lake
The first thing you need to do to get the hidden star is to swim to the bottom of the lake. This means you can just skip past the Undergrunt Gunner and head straight through the tunnel to the lake.
When you’re at the bottom of the lake, grab the green shell and shoot it at the ship wreck that is surrounded by spiky underwater mines. When you shoot the shell at the wreck, the mines will explode the wreck and reveal a Star Launcher. The Star Launcher will shoot you into the sky towards a purple and gold box.
Power Star Boo
When you land inside the box, you’ll be greeted by a Boo and an Urchin. The Boo has a Power Star so you’ll need to figure out a way to trigger a spotlight to defeat him. On the backwall, use the spinning poles to jump up to the gravity arrow and then spin it so the gravity is now heading towards the left wall. When you’re on the left wall, groundpound the spring to jump up and spin the gravity arrow again. Whilst you’re making your way around this room, make sure you keep an eye on the Boo and the Urchin as they’ll make their way towards you in an attempt to hurt you.
When you’re on the back wall, you’ll need to use the spring to jump towards the gravity switch. You should time your jump so that you land into a bubble, then when the bubble is near the gravity arrow spin to flip the gravity arrow. On the right wall you’ll need to spin attack the crystal to reveal a spotlight. You can then lead the Boo into the spotlight by spinning him into it. When you’ve successfully defeated the Boo, you’ll need to use the spring jump to flip the gravity arrow back to default so you can grab the Power Star.