Here is the full list of all 34 METAL SLUG XX achievements.
Finish the game on Difficulty HARD
Free all POWs in any mission
Clear all Special Training missions with Rank A or above.
Reach Rank 1st RECORD TIME on BEGINNER difficulty. Must play through all stages without quitting game.
Clear the game within 45 minutes. Must play through all stages without quitting game
Clear a mission with all Ikari members
Get the rare gem item
Defeat Rebel Gigant (Mission 6's boss) with at least half life
Get in all Slug vehicles
Rescue Rumi Aikawa
Collect the doll item
Collect 500 coins
Find the Old Man
Obtain all other achievements
Shoot down 30 enemies with Marco's Special Punch Action
Destroy 5 vehicles with Tarma's Special Action
Destroy 30 vehicles with Ralf's Special Action
Throw 30 enemies with Clark's Special Action
NNullify 100 enemy projectiles with Leona Heidern's Special Actionullify
Clear all 3 hidden paths
Clear all Target Elimination missions with Rank A or above.
Clear all Base Assault missions with Rank A or above.
Clear all POW Rescue missions with Rank A or above.
Defeat The Union (Mission 5's boss) 's parts in a specific order
Reach Rank 1st RECORD TIME on NORMAL difficulty. Must play through all stages without quitting game.
Reach Rank 1st RECORD TIME on HARD difficulty. Must play through all stages without quitting game.
Clear a mission with all Regular Army members
Defeat 40 future soldiers with the Zantetsu sword
As Eri, kill 10 enemy soldiers at the same time with grenades
Trigger the 100 Morden Soldiers encounter in Mission 2
Play the Combat School survival mission
Defeat 10 enemies with POW Hyakutaro's energy balls
Shoot down 20 UFOs with the Laser beam
Let a chicken hatch, grow, lay an egg and see a new chick hatch