Good bonus points
Do the color combos. (they'll tell you how to do it in the game)

How to get good weapons and lots o money
The best way to get lots of money and good weapons go to the item world with at least 10 characters that are strong. And please got to weak weapon first.(you'll still get the money and the good weapons)

Hidden Switch
If you go behind the throne in the overlords castle and fress X it will ask you if you would like to press a switch. Currently i have no idea what that switch does but i hope to find out soon enough

Another switch
There is another hidden switch in the skull in between the weapon and armor sellers at the beginning of the game.

Selecting Monster Character with Little Mana
After defeating a monster it will show up in Character select. It will be selected only with lots of Mana. In order to get the Monster with little Mana, go where you can fight the Monster, and keep defeating him until its Mana cost drops to where you can get it at your cost. Every time you defeat it go to Character select and watch the mana drop.

Level up characters and weapons quickly
To quickly get your characters and their weapon levels up, go to a stage that has a large amount of 'Invincibility' geopanels such as Episode 5, Stage 3. In stages like that you can manipulate the enemies onto the non-invincible geopannels and hammer away on them without having to worry about taking any damage. Once the enemies have become too weak to derive any real experience from, you can lift them and toss one onto the other to combine their levels, making one high leveled monster. Repeat the drill until your characters and weapon levels are where you want them.

Etnas Journal
If you hit the two switches (the one behind the throne in the overlords castle, the one in the skull between the weapon and armor sellers) you will unlock Etnas journal.

Quick level gaining
I have a really quick way to gain levels. This method is only available after you reach a certain point in the game. After you've reached Episode 5, Scene 3, you can use this method (Jotunheim, Terrible Cold).
This board is made up of all invincibility tiles besides one green one in the corner near the invincibility triangle.
Throw all the enemies on top of each other (increases the level of the monster, but also increases the exp). When one really powerful monster remains, lure it over near the green tile. Throw the enemy onto the green tile and trap it there with your characters and attack until all its life is gone. It will be unable to damage you because you are invincible, but you will get the exp for killing a monster that is level 112!!
Another quick way is to perform this after summoning the committee and asking them to triple EXP

Unlock Majin Class
To unlock the majin class, you need each of the following classes, at level 200: warrior, male brawler, ninja, rogue, and scout. The majin is the best class in the game