Here is the full list of all 15 Metal Slug X achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
Save 200 POW's
Kill 2000 soldiers
Play with a friend
Finish game without using continue
Clear Game
Save 50 POW's
Clear Mission 5
Throw 200 grenades
Clear Mission 4
Clear Mission 3
Collect all weapons
Kill 200 soldiers
Throw 50 grenades
Clear Mission 2
Clear Mission 1