This boss will be the easiest of the three. For preparation only take Frankenimpa with you. If you have collected all the treasures in the game your Frankenimpa will be powered by and you can take 30 of them into battle with you.
Drop down into the area of the boss and immeditaley run into one of the stepping stones to lift up a block in front of you, this will block the attack of the dragon. With his first attack he will start to breath fire where you are stood behind the block, he will then start to turn.
Immediately run around the block in front of you and throw a couple of you minions at the dragon and it will stun him. The moment he is stunned, send all of your minions to attack. If you have done this quick enough the dragons life bar will be drained before he can recover.
This will give you your final piece of the compass completing the story of the game. You can continue the level select after the game if you have missed any achievements.