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.
Play with a friend
Finish game without using continue
Clear Game
Save 200 POW's
Kill 2000 soldiers
Clear Mission 5
Clear Mission 4
Clear Mission 3
Throw 200 grenades
Collect all weapons
Save 50 POW's
Clear Mission 2
Throw 50 grenades
Kill 200 soldiers
Clear Mission 1