Battlerock Galaxy is accessible from the Fountain observation dome. You need 12 Stars to enter.
Star 1: Battlerock Barrage
Star 2: Breaking into the Battlerock
Star 3: Topmaniac and the Topman Tribe
For the time being, ignore the Hungry Luma you meet on this level. In this level you will reach a Luma Shop to buy either a 1-Up Mushroom or a Life Mushroom for 30 Star Bits. The Life Mushroom might be useful if you need help during the upcoming boss fight. Boss fight: Jump on Topmaniac then spin him into the force field. Repeat twice more.
Star 4 (Daredevil Comet): Topmaniac's Daredevil Run
Available when a Prankster Comet is in this Galaxy. This is a repeat of the boss fight from Star 3, except you only have one life point.
Star 5 (Hidden): Battlerock's Garbage Dump
Enter Star 3. Proceed through the level as for Star 3. When you meet the Hungry Luma, feed it 30 Star Bits. You will need to throw bombs (by performing a Spin Attack) at the garbage to destroy it all within 30 seconds. If you can throw the bombs so that they land near the yellow spots on the floor, you should be able to destroy all the garbage in time. The yellow spots, by the way, are coins embedded in the floor which you shoot Star Bits at to release.