Here is the full list of all 12 Galaga achievements worth 200 gamerscore. It takes around 1-2 hours to unlock all of the achievements on Xbox 360.
Shoot down all enemies in a Challenging Stage!
Shoot down a red captured Fighter!
Find the Blue Spaceship during battle!
Play up to Stage 30!
Play up to Stage 20!
Find the Mosquito during battle!
Find the Galboss during battle!
Find the Dragonfly during battle!
Play up to Stage 10!
Find the Stingray during battle!
Form a Dual Fighter!
Find the Scorpion during battle!