Unlocked after finishing the last mission in Chapter 7 (Dark Cloud Rising)
At the end of the last mission you will have to fight an airship.
Try to have a few rocket launchers ready and some ammo
When the airship is high in the air, shoot the rocket launchers in between the 2 fans on either side (the resulting explosion will hit both fans and do bonus damage for exploiting the weakness)
From time to time the airship will fly lower to drop off reinforcements.
At this point just ignore the reinforcements and try to hit the fans in the airship's wings as fast and as much as possible (Maybe use some ragnite/magic since they can hit hard, and 'pauze' the game for a few seconds)
Once the airship flies higher, try to kill the reinforcements with normal attacks (to replenish mana, this avoids the use of items if you like it that way)
As long as you have ammo for rocket launchers this is the moment to use them. If you run out of ammo just hold out and try to avoid the AoE attacks
Repeat as needed.
Remember, the weak spots are the 2 fans in the left and the 2 fans in the right wing.