Here is the full list of all 44 GIGANTIC ARMY achievements.
Destroy 100 enemies.
Destroy a boss using the MJ-88 Missile Pod.
Stomp a land mine.
Clear the game in Easy Mode.
Clear the game on Normal Mode.
Clear the game on Normal Mode with no continues.
Beat Stage 1 on any difficulty mode and with no continues.
Beat Stage 2 on any difficulty mode and with no continues.
Beat Stage 3 on any difficulty mode and with no continues.
Beat Stage 4 on any difficulty mode and with no continues.
Beat Stage 5 on any difficulty mode and with no continues.
Beat Stage 6 on any difficulty mode and with no continues.
Clear a stage on Easy Mode without taking damage.
Clear a stage on Normal Mode without taking damage.
Clear a stage on Hard Mode without taking damage.
Clear a stage on Insane Mode without taking damage and with at least 140 seconds left on the timer.
Unlock Hard Mode.
Beat the True Last Boss on Hard Mode.
Unlock Insane mode.
Clear the game on Insane Mode.
Play a stage in Practice Mode.
Achieve a final score of over 4,500,000 on Easy Mode with no continues.
Achieve a final score of over 9,000,000 on Normal Mode with no continues.
Achieve a final score of over 13,500,000 on Hard Mode with no continues.
Achieve a final score of over 18,000,000 on Insane Mode with no continues.
Achieve a final score of over 2,000,000 on Easy Mode.
Equip at least 3 different combinations of main, sub and special weapons.
Destroy 10 enemies with the SKB-BS Pile Bunker in mid-air.
Have your precious and expensive Type S-95 SEC Shield destroyed.
Destroy an end of stage boss with no special weapons.
Destroy a boss using the GDW-76 Beam Cannon.
Destroy 100 enemies with the ZAT-77 Assault Rifle.
Destroy 100 enemies with the SRI-U6 Riot Gun.
Destroy 100 enemies with the UG-M Grenade Gun.
Destroy 500 enemies.
Destroy 1,000 enemies.
Destroy 10,000 enemies.
Destroy 100 enemies with the SKB-BS Pile Bunker.
Destroy 3 or more enemies with one shot from the SHAD-51 Cluster Bomb.
Use your Vernier thrusters for more than 5 minutes in total.
Spend five seconds or more in the air (in one go).
Travel 10 km using the Dash Device.
Block enemy attacks over 100 times with the Type S-95 SEC Shield.
Run into an electric grid in Stage 6.