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