There are 30 Missions to complete to earn this. If you follow these tips you'll coast through the game. You do not need any side missions to beat the game.
-Talk to all villagers for clues to the 'trick' of the level. There is a level that has no monsters for instance and you must find the monster sanctuary to gain any levels.
-Before you pay to reset time to 30 seconds grind levels outside a town to get maximum cash gain.
-If in a HP pinch do not suicide into the enemy if you have allies with projectiles. You can usually abuse these allies as you stand away from the damage and they slowly hack away at the enemies.
-Bugs can be killed with the flyswatter instantly. There is a weapon that destroys dragons fast. Figure out who your enemy is, you may have a tool to perform an easy OHKO.
-Grinding levels till You > Evil pops up is always a sure fire way to win a stage.