Build up throughout the game - Will unlock on Level 15 or later
Each time you kill an enemy or free a caged animal, you'll get XP which are represented by yellow orbs. Each time you pick one up, you'll see the circle closing around the Mini Ninjas logo located on the bottom left of your screen. Once the circle is fully closed, you will level up. So, kill as many enemies as possible, free as many caged animals as possible, and you will unlock the achievement once you've leveled up 15 times. The circle around the Mini Ninja logo will become red when fully leveled up.