This will be part of a turorial fight very early in the game, so this probably even is a story related cheevo.
Iddhi is gained attacking enemies, the bar at the top will fill up with every attack and clean block as well.
To use it press the cn_RB button and one characters attack button (cn_Acn_Bcn_Xcn_Y)
If you have more than one bar you can quickly press the characters button again to use another bar.