Disgaea: Hour of Darkness Guides and Walkthroughs
The third map on Cave of Ordeals is the best power leveling location in the entire game. Here you will find 9 of the most durable enemies that you can come across standing on 50% bonus EXP tiles. They have insanely high Defense (decreases melee damage) as well as Resilience (decreases spell damage) which means that if your characters are under leveled your chance of taking out these guys is going to be practically 0.
Prior to purchasing any upgrades the enemies on this map are all Level 150 with the main guy in the center being Level 160. Unless your characters are very close in level to the Galactic Demons you face on this map I wouldn't recommend grinding here. Also, if your characters don't have AoE spells which cover 3x3 areas I would recommend leveling their weapon/casting skills up on different maps first.
What I typically like to do for this map is use a Ninja/Monk who has a high enough Fists rank to have Big Bang unlocked. This move will allow you to hit all the enemies on the board in one go and, assuming your Ninja/Monk is high level, severly damage them all. Then you can pick the enemies off that are low on health on characters which aren't as powerful as your Fist user.
As you could probably already imagine, leveling up your casters on this map will be easier than most of your single target damage dealers. If you need to level up individual spells so you can cast bigger AoEs I strongly recommend the Forest of the Dead map - it's the best map for lower level and poorly geared casters to get EXP.