You will get this achievement when you beat the game by defeating King Thorn.
Doing so is not as difficult as it first appears. First, equip the Eye of Strength, this will increase the strength of your attacks. Now, mercilessly batter Thorn with Rubble's Stone Storm attack. Every so often, Thorn will summon other enemies - you should ignore them and focus your attacks on Thorn. The other enemies will disappear as you beat on him.
If your health starts to get low, equip the Eye of Restoration, switch to Kameo, and fly around a little while to restore your health. Be sure to re-equip the Eye of Strength, then go back to Rubble and keep beating on Thorn until he goes down.
At this point, Kalus (Kameo's evil sister) will come to try and resurrect Thorn. Switch to Chilla and fire ice spikes into her until she goes down. Ignore all enemy attacks, they won't affect your aim.
That's it, you've beaten the game, congratulations!