Quest Complete Achievement
Completed the game in Hard mode.
Start the game on Hard. You can play in either Arcade or Home mode. Then just complete the game (kill the dragon) before you run out of lives.
Difficulties do not stack (as there is no achievement for completing the game on Easy), so you must play through the game at least twice so you can unlock "Lair King" on Easy. You cannot unlock "Lair King" if you play on this difficulty (Hard mode is missing one point-awarding move [the first whirlpool in the Raging Waters scene], so you can’t get a perfect score).
This achievement can still be unlocked if you lose all your lives and choose to continue playing the game.