Battlerock Galaxy is accessible from the Fountain observation dome. You need 12 Stars to enter.
Star 1: Battlerock Barrage[edit]
You start at a set of metal saucers. Collect the Blue Star Chips to make a set of Pull Stars appear, which you can use to get to the Launch Star. Take it to arrive at another saucer. Go to the other side and jump on the spring to get to another saucer. Go to the other side again and spin the screw to activate everything. The saucer starts moving, and the cannons start firing. As the saucer moves, dodge the cannonballs and electric fences until it stops moving. Jump to the big saucer. You can use the Bullet Bills to break the glass cages on the saucers. Position yourself so that the glass cage is between yourself and the Bullet Bill. Then move so that the bullet hits glass and not you. One saucer has coins and Star Bits, and the other one has the Star in a cage. Break it with a bullet.
Star 2: Breaking into the Battlerock[edit]
There is a hidden star on this level. For the time being, ignore the Hungry Luma you meet.
You start on the saucers again. This time, the Launch Star is in a cage, and there are bomb dispensers on the ground. Touch a bomb to pick it up and light its fuse. Don't hold it for too long, or it will blow up and you will take damage. Also, don't be too close to it when it explodes. Spin to throw it and break cages. You can destroy the cages with star bits if you want, then destroy the cage with the Launch Star. Launch to a planet that is made up of multicolored spheres. There are Chomps rolling around, but you can destroy them with bombs or with the Rainbow Star in a cage. When you're ready, use a bomb to blow up the cage with the Luma, who transforms into a Sling Star for you. Take it, then take the Launch Star it propels you into.
The next section consists of many Pull Stars and mines. Use the Pull Stars to navigate around the mines and into the Launch Star. From where you land, hit the red switch for star bits if you want. Enter the pipe to move on. There are more bomb dispensers here, but these bombs try to walk towards you. Spin one to stop it, then pick it up and throw it at the cage and wait for it to explode. Go into the square hole, which is actually a cannon. Point the remote at the screen to aim, then press 
to shoot. Try to shoot the big crystal to free the Luma, which turns into a Launch Star. Shake as soon as the Launch Star appears to take it without having to use the cannon again. If you miss, you'll need to blow up the cage and shoot the cannon again.
You land on a rocket ship-like planet. Pick up the bomb, then jump over the electric bars and make your way to the cage at the end. Blow it up and get in the cannon. Wait for the moving platforms to get out of the way, then shoot into the Power Star.
Star 3: Topmaniac and the Topman Tribe[edit]
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. The life mushroom gives you three extra slivers of health.
Boss fight: Topmaniac. Jump on Topmaniac then spin him into the force field. Repeat twice more.
Star 4 (Daredevil Comet): Topmaniac's Daredevil Run[edit]
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 (Purple Comet): Purple Coins on the Battlerock[edit]
Available when a Purple Comet is in this Galaxy.
You need to collect 100 Purple Coins, while traveling through the level on an automatically moving saucer. There is no time limit but you will only get one chance to obtain most of the coins, so the pressure is on.
Follow the path of the coins, moving around and above and below the saucer and jumping as appropriate, while avoiding the electrified walls, rotating spheres on the saucer, and the balls that get fired at you. You'll need to jump over a couple of electrified walls, the first of which is an easy jump, but the second will require a Backflip.
Once you have all 100 Purple Coins, collect the Star.
Star 6 (Hidden): Battlerock's Garbage Dump[edit]
Enter Star 2. Collect at least 30 Star Bits from the flying saucers at the start. Proceed through the level as before until you reach the Hungry Luma. Feed it 30 Star Bits to transform it into a new planet, and use the new red Launch Star to fly to this planet.
Here you will be asked to clear all the garbage, which you will need to do within 30 seconds. To do this, pick up bombs then throw them (by performing a Spin Attack) at the garbage. As time is short you will need to throw the bombs such that they will do the maximum amount of damage. Helpfully, there are some yellow spots on the floor, and if you can aim throw the bombs so that they land near these spots, you should be able to destroy all the garbage in time. The yellow spots, by the way, are coins embedded in the floor which can be released by being shot at with Star Bits.
Once all the garbage has been destroyed, collect the Star.
Star 7 (Hidden Luigi, Green): Luigi under the Saucer[edit]
This is one of the Hidden Luigi Stars, available after Luigi sends you a letter with a photograph of this galaxy. In addition, this is one of the three Green Stars, which you need to collect to unlock the Trial Galaxies.
Enter Star 1. Proceed through the level exactly as for Star 1 until you get right to the end. Instead of collecting the Star on the top side of the saucer, release Luigi from the glass cage on the underside of the saucer, using Bullet Bills as before. Talk to Luigi and he will give you the Green Star.