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.
Without forming a Dual Fighter, get a Perfect Bonus on a certain Challenging Stage!
Escape from a Boss Galaga's tractor beam!
Get a 70% or higher hit rate on Stage 10 or above!
Get a Perfect Bonus on the Stage 31 Challenging Stage!
Find the Blue Spaceship that appears in battle!
Reach Stage 30 and view the stage emblem!
Get a Perfect Bonus on the Stage 27 Challenging Stage!
Get a Perfect Bonus on the Stage 23 Challenging Stage!
Reach Stage 20 and view the stage emblem!
Get a Perfect Bonus on the Stage 19 Challenging Stage!
Find the Mosquito that appears in battle!
Get a Perfect Bonus on the Stage 15 Challenging Stage!
Find the Galboss that appears in battle!
Get a Perfect Bonus on the Stage 11 Challenging Stage!
Find the Dragonfly that appears in battle!
Reach Stage 10 and view the stage emblem!
Find the stingray that appears in battle!
Shoot down a red captured Fighter!
Form a Dual Fighter!
Find the Scorpion that appears in battle!