Maximum Firepower Achievement
Using a catapult, fling a knight with maximum power
In the level "Flowery Field" (the level after the giant bat boss), you will come to a point where you have to launch yourself out of a catapult. In order to obtain this achievement, you have to hit X when the power meter is full. This will launch you as far as possible. In order to get this achievement, hit the X button a moment before the power meter hits the top, and the delay between hitting the button and the game registering it will be fine. Don't click X too early or too late. It may take a few tries to get the timing down right, but it's not too hard.