If you play in the 'Save The Cobits' mode on the 1st moon, there is a mini-boss which looks similiar to aplant spitting lethal bombs at you. When you defeat it, it's head falls off. Knife it repeatedly to earn HP-ups.
Also, just for fun (and a challenge), when your Weapon's Bar/Gauge isnt flashing, double-tap the button to use its special. Try to kill all your enemies like this.