Here is the full list of all 15 METAL SLUG 2 achievements.
Save 200 POWs
Kill 2000 soldiers
Save 50 POWs
Clear game (Final Mission)
Clear Mission 5
Throw 200 grenades
Collect all weapons
Clear Mission 4
Throw 50 grenades
Clear Mission 3
Kill 200 soldiers
Play with all characters
Clear Mission 2
Clear Mission 1
Finish game without using continue