Hello!

After the mekadragon fight, you have to go to a room where you hit a giant crystal and ghost ninjas appear. When you get to the crystal, do the airstrike maneuver (down + attack) repeatedly. You'll be bouncing on this thing until the feat pops. It really is as easy as it sounds.

Here's a bonus tip: the air strike is also the best way to fight the ninjas (provided they are not already in the air). They usually stay on the ground, so it shouldn't be too much of an issue. You can air strike off one ninja and onto another one. They have no defense against it.

You can also use upward slash of your down+attack to instantly cut them while they fall down.