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Here's the official Walkthrough:
1. TRAVEL TO THE GALAXIES:

The Comet Observatory (initial zone) have multiple "rooms" in the shape of a house.
All of them require to fill first the Grand Star from the previous galaxy except from the "Observatory room".
The Observatory room is right aside Quick Revive, it will require $1250 points to open it and a Launch Star will take you to the Good Egg Galaxy.

Here's the order and location of each room:
- Observatory Room (at the left of Quick Revive) ---> Good Egg Galaxy
- Grand Star open the Fountain.

- Fountain (right aside Timeslip perk) ---> Honeyhive Galaxy
- Grand Star open the Kitchen.

- Kitchen (open the door at the left when start the map and go up the caracol stairs) ----> Fortress Galaxy
- Grand Star open the Bedroom.

- Bedroom (right aside Speed Cola) ---> Ghostly Galaxy
- Grand Star open the Machinery room.

- Machinery room (the room at your left when use the jump pad from Jug planet) ---> Toytime Galaxy
- Grand Star open the Garden.

- The Garden (at the very top of the Comet Observatory) ---> The Garden.
- Here's the final Grand Star that unlocks the buyable ending.


2. OPEN PACK A PUNCH:
You need to find 4 big golden coins around the map. The locations are:

- In the middle of the Comet Observatory, just in front of the Power Switch.

- On top of the Brown Planet behind the Cannon Planet in Good Egg Galaxy.

- On the top of the tree at the left when you arrived Honeyhive Galaxy. It's easier to shoot from the Tree House on the same planet (zone wih Vigor Rush perk).

- At the left of the damaged rocket on Fortress Galaxy.


3. POWER & GRAND STARS LOCATION:

There's 3 Power Stars per galaxy that need to be fill with zombies kills to access the galaxy's Grand Star. Their locations are:

GOPD EGG GALAXY:
- In the middle of the Star Planet.
- In the back of the cannon in the Cannon Planet.
- At the top of the Mountain Planet, you can find a wallbuy in front of the star.

Once complete a fence door open a Launch Star where Deadshot Daiquiri is that tales you to the Capsule Planet in the middle of the galaxy.
You must go all the way up and will find the Grand Star at the top.

HONEYHIVE GALAXY:
- In the fountain at your right when you arrived the galaxy.
- On the Tree House near Vigor Rush at the top of the first planet.
- At the back of the Pillar Planet near a Wunderfizz and Mystery Box location.

Once complete a fence door open a Launch Star at the bottom of the Queen's Planet. You will find the star at the top of the planet where the big leaf is.

FORTRESS GALAXY:
- In front of Stonecold Stronghold perk, once you go pass the "hall" on the second planet.
- In the middle of the third planet, before the jump star that lead to Hungry Luma.
- At the very end of the galaxy.

Once complete a fence door open a Launch Star that lead the House Planet. The star is at the top of the house.

GHOSTLY GALAXY:
- At the center of the Graveyard Planet. Is the same planet where the Launch Star to exit the galaxy is.
- At the bottom of the stairs in the first floor of the Haunted Mansion planet.
- At the attic of the Haunted Mansion. You need to go through the fence with the arrow and Elemental Pop is at the right of the star.

Once complete a fence door open a circular planet in front where Hungry Luma is.

TOYTIME GALAXY:
- In the second floor of the first Toy Planet, at the center.
- At the top of the Cake Planet.
- At the center of the Cake & Candy Planet. Is the last planet on the galaxy where Widow's Wine is.

Once complete a fence door open a Launch Star in the Tootsie Pop planet that lead to the Robot planet. The Star is at the second floor.

GARDEN:
Once complete all the previous galaxies the top of the Observatory opens and it will lead you to the Garden. You need to fill the final Grand Star that require twice of the normal Grand Stars.
You can't return to the Observatory, so be prepare.