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.
Find the Stingray during battle!
Find the Scorpion during battle!
Form a Dual Fighter!
Shoot down a red captured Fighter!
Shoot down all enemies in a Challenging Stage!
Find the Dragonfly during battle!
Find the Mosquito during battle!
Find the Galboss during battle!
Find the Blue Spaceship during battle!
Play up to Stage 10!
Play up to Stage 20!
Play up to Stage 30!