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