Disgaea: Hour of Darkness Cheats

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.

NOTE: You can level your weapons by attacking allies on the invincible
geo panels as well.

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.

To get Vile Brilliance talk to Manty after Episode 11.

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.

Message: After defeating Overlord in (Chapter 6 - Writhing Shadow) go back to it, and there you will fight a Blue Dragon. If you have a Character or Monster you wish to level up, let them take the last hit to destroy Dragon, and they will go up one level each time it is defeated. Do this to each Character you want leveled up.