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