Successfully complete the game to unlock Gentleman's mode. This mode is a more difficult version of the game. You will overwrite your completed saved game and reset everything to 0%.
There is a secret level in Tealand. In order to reach it, you must go to Level 5-2 and work your way up the clock tower. Once you have reached the next area, continue upwards. There is an area that forces you to move right, accompanied by an arrow sign telling you to go up. Pass the sign, and fall into a small gap. Walk to the right and through the false wall. Continue climbing all the way to the top. There will be a large white chest, just like at the end of every stage. It will unlock Level 5-?, which is located in the clouds. Henry will begin the level in a small blimp.
Find the indicated hidden suit piece to get the corresponding effect:
This strategy can be used with any enemy, though it works particularly well for the majority of Bosses. Once you acquire the boomerang, do not trade it for another sub-weapon. The boomerang, when charged to its fullest level with puzzle combinations, will home in on the opponent and cause most to stop attacking.