Etna's diary
There are two hidden switches in the castle. One is behind the throne, and another is near the skull on the counter of the Rosenqueen shops. Flip both of them to allow access to Etna's diary, which is in the middle corner of the intermediate hallway between the throne room and the shops. If you visit this area consistently for each chapter, you will be rewarded with a Testament item in the final chapter. Information in this section was contributed by jimmi clancy.

Get the most out of your turn
If you are trying to level all of your characters mainly by using team attacks, etc., move all your characters into position for a team attack. As long as none of them besides the initial attacker have not performed any actions, after the team attack you can move them back to their original locations and move them again. Repeat this as desired. This also works if your characters are just out of a healers range; you can move them, heal them, then move them back into battle.

Easy experience
In stage 5, level 3 (Terrible Cold), there is only one Geopanel that does not have the invincibility attribute. Combine all the enemies by throwing them into each other, making one high level enemy. Throw the enemy onto the Geopanel that is green. Trap him/her on the Geopanel without invincibility by surrounding him with your characters. Since all of your characters are on invincible panels you cannot be defeated, yet you can wear down the high-level enemy who is trapped on the non-invincible panel. This is a great way to jump 10 levels at once early in the game.

Get to the Jotunheim episode and go to Terrible Cold area. The entire map will be invincible except for one location. Throw all of the Sexy Demons together. This should get you about a level 114 Demon. After that, throw her all the way to the corner at the top of the map that has the "None" status and surround her, one on each side. It helps if your attack is at least 300 on each of the characters that are attacking. It also speeds the process if you bring one or two Mages with Brave Heart. Keep casting that on your characters every turn, as it is stackable. This trick is worth the time, and gains about four to six levels for each character in the team attack.

In episode 9, level 1, there are only nine enemies in the level. They are in a 3x3 block. Use Mage's 9 square attack or Laharl's Winged Slayer attack to hit all of them at the same time. For any that remain, use your lower characters to finish them off. You should be able to kill all the enemies on your first turn, so you never get hurt. This usually works when you are between level 20 and 45 to 50.

Unlock Hidden Classes
Archer: Level 5 or higher in Bow Weapon Mastery.
Celestrial Host: Female Cleric, Rune Knight and Archer all at level 100 or higher.
EDF Soldier: Level 30 or higher Gun Weapon Mastery.
Majin: Male Warrior, Fighter, Ninja, Theif and Ranger all at level 200 or higher.
Ninja: Male Fighter and Male Warrior with a total level of 20 or higher.
Rune Knight: Female Warrior and Female Mage each at level 10 or higher.
Samurai: Female Warrior and Female Fighter with a total level of 20 or higher.
Strider: 2 Fighters/Warriors, Males or Females, each at level 5 or higher.
Theif: Both Fighter and Warrior, Males or Females, each at level 5 or higher.

Get Longinus
Get Level 25 Spear Mastery and talk to Longinus

Unlock Priere and Marjoly
Marjoly: Beat the last stage of Mysterious Seal.
Priere:Beat the last stage of Alt. Netherworld.

Unlock hidden maps
Alt. Netherworld: Get a high enough rank at the dark assembly, pass the bill.
Baal's tower: Beat all levels of Alt. Netherworld and Mysterious Seal.
Cave of Ordeals: Get a high enough rank and pass the bill for it.
Human world: Beat the human invasion chapters and pass the bill.
Mysterious Seal: Get a high enough rank at the assembly and do the bill.
Prinny Land: After the reincarnation chapter, pass the bill.