Here is the full list of all 15 METAL SLUG achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Free 100 POWs
Kill 1,000 enemy soldiers
Defeat a boss using the METAL SLUG ATTACK
Clear FINAL MISSION
Clear MISSION 5
Free 50 POWs
Throw 200 grenades
Clear MISSION 4
Clear MISSION 3
Throw 50 grenades
Kill 200 enemy soldiers
Clear MISSION 2
Clear MISSION 1
Play in MULTIPLAYER
Clear the game using no continue