This hidden star is obtainable during the Breaking into the Battlerock level in Battlerock Galaxy.
Blow up the Cages
You’ll firstly need to use the Bob-ombs to explode the cages. You’ll want to collect the Star Bits in the other two cages as well, as you’ll need those for later! To pick up a Bob-omb press Y, then to throw it press Y again. It will take a few seconds to explode and it needs to be in pretty close proximity to the cages to blow them up. After you’ve collected all of the Star Bits, use a Bob-omb to blow up the cage on the farthest saucer, to reveal a Launch Star. Use that Launch Star to take you over to the next area.
On the new colorful ball planet, there will be lots of Chain Chomps roaming around. You’ll need to speak to the Hungry Luma on the yellow ball who will ask for 30 Star Bits so they can transform. You’ll likely have around 20-25 Star Bits already if you destroyed the cages on the saucers. You can easily get a lot more on this planet by using the bob-ombs, which spawn on the green ball of the planet. You can either use the bob-ombs to destroy the Chain Chomps or you can throw them at the caged Star Bits. Both options will yield lots of Star Bits. When you have enough, head back to the Hungry Luma and shoot the Star Bits at it until it’s stuffed.
When it’s full, the Luma will transform and a new planet will appear. Use the Star Launcher that appears after the Hungry Luma has gone to take you to the new planet.
Clean up the Ammo Depot
When you arrive on the new planet, you’ll meet a Gearmo who asks you to help clean up the Ammo Depot. He tells you to use Bob-ombs to destroy all of the garbage. You might need to give this a few goes as you only have 30 seconds and the Bob-ombs take 10 seconds to explode, with new Bob-ombs only spawning when the others have exploded.
When you’ve cleared all of the garbage, the Gearmo will hand you a Power Star!