So, the Disgaea games are somewhat infamous for being "those games where you get to grind up to level 9999" and naturally they have a habit of giving you levels deliberately and obviously designed to help facilitate this. Here's one of the best known from the first game, level 5-3: Terrible Cold.
(It's actually massively obsoleted by most other levels designed for this, but most of those are really simple "here's a bunch of enemies on EXP +100% geo panels set up in a convenient punching bag formation" affairs that aren't nearly as interesting to talk about.)
The story of this level is that the chapter's villain is trying to defeat Laharl by taking advantage of his extreme weakness to sexy women, which is why all the enemies are sexy lady-type monsters. Disgaea's silly like that. But it doesn't matter all that much for the mechanics of the whole thing.
You'll notice the invincibility geo panels everywhere. They are, in fact, everywhere everywhere and cover every tile but one1. This means that you can deploy anyone you want and not have them be in any danger, thus allowing you to level anyone up safely. Pretty good grinding trick, right?
We can do better.
One of Disgaea's signature mechanics is that you can lift and throw anyone—ally, enemy, obstacle, whatever—with the only restriction being if there's a "no lifting" geo effect in play. Which there isn't here, in case you haven't noticed. Throwing enemies is how you're expected to win this, even, since otherwise it's basically impossible to get them on the non-invincible panel. But there's another trick, which is that if you throw an enemy at another enemy, they're combined into one enemy that has both their levels added together. Enemy experience rewards are based on how much experience the enemies need to level up, and naturally this number scales exponentially, so combining enemy levels has a massively outsized effect on how much experience you get. And here we have a level where you can survive any hit, no matter how strong...
So the trick is to find an enemy to combine everyone into. Ideally you use one of the catgirl-type monsters—the succubus-types have a move that drains your HP—and you need to find one who randomly has her defense decreased from the base value. Then you throw every enemy into her, and now you have a level 112 catgirl who ideally has low enough defense that you can actually damage her2.
Then you throw her onto the one tile that doesn't have invincibility, and have the character you're trying to level up kill her. Congratulations, you now have a character who's leveled up enough for a substantial amount of the game to come! And because money rewards are also based on enemy level, you can afford a pretty good weapon for this point in the game off the proceeds from just one round of this.
Obviously this isn't anywhere near the most effective grinding level in the game. It's blatantly gonna fall off the moment you unlock something that takes less effort to use. But again, this is just such a unique way to do level grinding that it's worth looking at anyway
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Okay, technically there's also your base panel, but using that one for this requires you to kill off all your characters in the process of destroying the base panel, which isn't even remotely cost-effective and becomes impossible later on.
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It's actually more effective to stop before throwing the last enemy in, leaving the catgirl at level 98—the formula is designed so levels past 99 have their experience requirements scale differently in a way that makes them have lower requirements, and thus lower kill rewards, until you're a ways into the hundreds—but it doesn't matter that much since you can't combine them into a monster that's exactly level 99. And I'm not sure how many "stronger enemies" bills you'd need to stack to be able to get that number off the top of my head.
