Here is the full list of all 29 Galaxy Reavers achievements.
Obtain 10 tactical devices
Boost a warship's level to 10
Win 100 battles
Obtain 50,000 Gold
Boost Commander's level to 5
Clear Stage 10
Win 30 battles
Obtain 4 warships
Boost Commander's level to 10
Boost Commander's level to 20
Boost Commander's level to 30
Boost a warship's level to 30
Boost a warship's level to 50
Boost a warship's level to 70
Obtain 8 warships
Obtain 12 warships
Obtain 50 devices
Obtain 200 devices
Obtain 500 devices
Obtain 50 tactical devices
Obtain 150 tactical devices
Obtain 300,000 Gold
Obtain 1,000,000 Gold
Clear Stage 27
Clear Stage 45
Win 300 battles
Destroy 150 warships
Destroy 500 warships
Destroy 2,000 warships