Here is the full list of all 20 Galaxy Warfighter achievements.
Go through the first boss.
Improve any equipment.
Complete 10 levels.
Kill the boss while time is stopped.
Kill 10 enemies with one blast.
Complete 50 levels.
Complete 100 levels.
Improve your armor to the maximum.
Improve protection to the maximum.
Improve drones to the maximum.
Improve the main gun to the maximum.
Improve the wave to the maximum.
Improve stopping time to maximum.
Earn 1000 coins.
Earn 10,000 coins.
Earn 50,000 coins.
Kill 1000 enemies.
Kill 5000 enemies.
Kills 4 bosses in a row without going to the main menu and without losing even once.
Kill 10000 enemies.