It's good to have a few guys that are stronger. I kept Laharl Etna and Plenair my strongest when leveling. I found the most important is get your weak guys in on group attacks with all your strong characters. You can get a few levels a turn canceling the movement after the group attack and getting them in on one with another guy. As for the huge damage... need to find the rare items get the right item residents moved onto them then level them up. It's much harder to level in this one compared to disgaea 2. Disgaea 3 I found even easier then 2 to get the levels up.