Most humanoid classes are unlocked by reaching certain levels of weapon mastery with particular weapons on any character, then passing a bill (usually named something like "Make a X").

To unlock new monster, defeat them in battle. Killing more of the same monster reduces the mana cost at creation, up to a cap at 30 kills (this is why the number of monsters killed is shown at creation).

You unlock new humanoid classes by meeting certain requirements (reaching a certain level in Weapon Mastery in X and/or Y, capturing an enemy, setting off a big geo chain, etc) AND by passing their respective bills in the Dark Assembly.
For monster units, IIRC you just need to encounter and defeat them.

If you meant unlocking new tiers (though I find it a bit unlikely), you just need to reach a certain level with the tier immediately below it. As such, you can't just level a tier 1 unit to lv 200 and expect to unlock all other tiers like in D1, you need to reincarnate/create a unit of the next tier and level that one up.

Just look up "Disgaea 2 classes". There are some simple wikis that explain what the requirements are.

There are lots of classes that you'll naturally unlock, but some require specific conditions (like having a character with lvl 3 bow-skill to recruit an Archer girl. Other classes require you to be pretty much done the main story before they're obtainable.

I have all the regular ones. Check the faqs to see requirements. Majin slow and powerful and like never dies. Heavy tank chars like never die. Bow chars weak

Originally posted by Z3n:

Most humanoid classes are unlocked by reaching certain levels of weapon mastery with particular weapons on any character, then passing a bill (usually named something like "Make a X").

To unlock new monster, defeat them in battle. Killing more of the same monster reduces the mana cost at creation, up to a cap at 30 kills (this is why the number of monsters killed is shown at creation).

I'm pretty sure that is not a thing with monster creation in this or future Disgaea games (killing a monster to reduce the mana cost to create it). It was something that was only done for the first game. Creating higher tier monsters from 2 onwards means getting a lower tier monster to a certain level, much like with humanoids.