Here is the full list of all 21 Galaxy Bricks achievements.
Break 100 bricks
Used a powerup 20 times
Completed 10 levels
Defeated a boss
Defeated 100 Invaders
Break 10 bricks
Completed 5 levels
Used a powerup 5 times
Broke a brick
Use a powerup
Completed a level
Completed 20 levels
Defeated 5 bosses
Defeated 10 bosses
Break 1000 Bricks
Used a powerup 1000 times
Defeated 500 Invaders
Defeated 1000 Invaders
Completed Standard Campaign
Completed Doodle Campaign
Completed Retro Campaign