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