Beating the post game story unlocks some cool things, including the original Disgaea trio, Laharl, Flonne and Etna. However, beating these characters can be very tricky in Disgaea 6: Defiance of Destiny. Thankfully, like all tactical games, this one isn’t too bad.
While the core set up can be similar to Baal, these fights can also be cheesed rather easily. With this in mind, I would actually suggest building a couple damage dealers and a few thieves.
Damage dealers just need every perk boosting damage. Doesn’t matter how or why, just make sure it boosts power. From there, Laharl loves to counter attack, so anything that boosts that is going to be invaluable. The rest comes down to strategy and luck.
Even though this covers multiple stages, the core tactic is the same. Send thieves out to throw prinnies. The goal should be to grab one, toss it at as many as possible, and watch them all explode. If you do it right, this should make it so you’re only really fighting Laharl, Etna and/or Flonne.
There are two ways to really approach this problem. Either burn Laharl/Etna in a turn or take out Flonne. Out of the three, she is the most deadly, because she will heal Laharl back to full health and his Evility will eventually make him too powerful if you can’t already burn him.
Outside of this, play around with how you move. I found sometimes if I moved a specific way, Flonne would not come support her partners as fast, giving me an extra turn to kill Laharl and/or Etna.
And, if all else fails, just grind a bit more. It shouldn’t take more than three reincarnations at level 9,999 to defeat Laharl.