Disgaea: Hour of Darkness/Levelling

To begin, it's best to create characters when the creating character has 100 Mana or more, in order to boost the starting stats of the new character. This doesn't really apply earlier in the game, as you just want some characters to fill space and do some damage (and hopefully unlock more classes as they gain levels). Additionally, spend some money to buy some equipment (or use some equipment levelled up and modified (with the appropriate Specialists) for the purpose) for the new character to use in the process.

Next, save the game and go through as many Promotion exams as possible with the character (saving between each exam in order to prevent losing any gained levels when the character inevitably fails an exam). After a handful of exams, it's time to head off to the item world for a quick trip through the character's lower levels.

Item World
Choose an item you'd like to level up (or from which you'd like to retrieve some good residents). Statisticians (EXP) and Armsmasters (weapon mastery) are highly important to have when trying to level up characters, so they're also good to go looking for when leveling up earlier in the game. Save the game before entering in case you encounter a problem early on, as quitting and restarting the Item World for that item may be more beneficial than exiting (since the Item World is random, things will be different when reloading the game from a save prior to entering the item).

Skills that move enemy characters are of vital importance when trying to get through levels in the Item World. When trying to level new characters, check each level for experience bonuses and just skip them if possible when the experience bonuses aren't there or can't be received (for example, if you can't get enough of a geo chain bonus to acquire the experience bonus). Run (or throw characters) across the level and move the Gatekeeper with one of the listed skills to get through it with as little interaction with the level's enemies as possible. When an experience bonus is found, bring out the characters that need experience before the last enemy goes down (if the geo chain is likely to kill all of the enemies and there are safe points, bring the characters out to safe points before setting off the chain). Kill the last enemy with the experienced characters (or give the new character a strong weapon and let him/her take the finishing blow, if possible) and reap the rewards of the bonus.

Transmigration
When the newly-levelled character has enough mana, transmigrate (to the next class tier up if available) and start again. 175 Mana is generally best early on, as this gives the "Average" option in the transmigration.

Transmigration returns the character to level 1, but you get the chance for a significant bonus to the character's starting stats, as well as the chance to change their class (which can also boost their stats). Story characters can also be transmigrated, though they can't change class. Later on in the game, it's more important to use the higher level transmigrations that require more mana (and possibly Assembly approval), as these give higher bonuses for the starting stats.

Bonus Maps
Cave of Ordeals 3 and Demonhall Mirror, once unlocked, are excellent maps for gaining the most possible experience in the shortest amount of time. With well-developed weapons (with high level Statisticians) and "Make Enemies Stronger" bills, CoO3 becomes a great place to level to 2000, and Demonhall Mirror can take the characters to 8000.