Full Moon
One hidden cheat lets you skip the game's Full Moon level. Start playing the level and then exit the game or quit back to the map. When you restart play, the game indicates you have completed the Full Moon level, which saves you time and effort, and lets you move on to other parts of the game.
Insane
Successfully beat the game with any character to unlock the Insane difficulty level. On Insane difficulty, enemies possess roughly 10 times more health and inflict three times as much damage. Completing certain game levels on Insane difficulty also unlocks playable characters such as the Cult Minion, Necromancer, King and the Open-faced Gray Knight.
Low Levels
While competing on the low skill level, travel to the Thieves' Forest. Kill the first few thieves you encounter to send the rest scurrying away, which in turn summons a large troll; the large troll spawns smaller trolls. Hang around and kill as many of the small trolls as you want to build up experience points and upgrade your level. Avoid hitting the large troll to keep it alive, allowing it to continue producing the easy victims.
Middle Levels
After your character is upgraded to at least Level 25, visit Lava World and follow the walkway until you reach a metal wall. As long as you don't break the metal wall, fire demons will continue to spawn, allowing you to load up on easy experience points earned when you kill the monsters. Likewise, the Marsh also provides easy victims, with another large troll similar to the one found in the Thieves' Forest. Kill the spawned little trolls to level up with ease.
High Levels
The aforementioned tricks don't work well after reaching Level 40 because they don't provide enough points to easily upgrade levels. After reaching this level, the best opportunity to rack up experience points comes on the last level against the Painter boss, who has a toolbox for a head and makes paintings that attack you. However, the boss won't attack you until half his life is drained. Take advantage and use your strongest attacks to gain experience early on against him. After defeating the painter, avoid breaking the purple crystal that summons the next boss and instead leave the room the way you came in, which makes it possible for you to fight the Painter again and earn more experience points.