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