Here is the full list of all 20 Zombie Hunter: D-Day achievements.
Upgrade firearms 20 times.
Unlock 3 firearms.
Upgrade firearms 10 times.
Complete the stage D-Day.
Complete the stage D-60.
Complete the stage D-40.
Complete the stage D-20.
Complete the stage D-10.
Achieve Level 10.
Achieve Level 20.
Achieve Level 30.
Achieve Level 40.
Achieve Level 50.
Upgrade firearms 30 times.
Upgrade firearms 50 times.
Unlock 5 firearms.
Unlock 8 firearms.
Upgrade 4 firearms to MAX level.
Upgrade 7 firearms to MAX level.
Upgrade 10 firearms to MAX level.