Here is the full list of all 20 Arcade Game Series: GALAGA achievements. It takes around 2-3 hours to unlock all of the achievements on Windows.
Escape from a Boss Galaga's tractor beam!
Get a Perfect Bonus on the Stage 31 Challenging Stage!
Get a Perfect Bonus on the Stage 23 Challenging Stage!
Shoot down a red captured Fighter!
Get a Perfect Bonus on the Stage 19 Challenging Stage!
Get a Perfect Bonus on the Stage 15 Challenging Stage!
Get a Perfect Bonus on the Stage 11 Challenging Stage!
Get a Perfect Bonus on the Stage 27 Challenging Stage!
Without forming a Dual Fighter, get a Perfect Bonus on a certain Challenging Stage!
Get a 70% or higher hit rate on Stage 10 or above!
Find the Blue Spaceship that appears in battle!
Reach Stage 30 and view the stage emblem!
Reach Stage 20 and view the stage emblem!
Find the Mosquito that appears in battle!
Form a Dual Fighter!
Find the Galboss that appears in battle!
Find the Dragonfly that appears in battle!
Reach Stage 10 and view the stage emblem!
Find the stingray that appears in battle!
Find the Scorpion that appears in battle!