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.
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.
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.