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