My issue, to be fair, was trying to have one of every class. That was probably my oops. ;3
Save this sort of thing for the post game when you can quickly raise level 1s into the hundreds with ease.
Generally for a Disgaea story campaign stick to a small elite group of characters comprised of 4 to 5 characters, with very specific roles, a Healer, a Mage, a Gunner, A Spear user, and a Sword user.
It's actually much less frustrating to concentrate on a small elite group of characters then spread your resources to two dozen.
There are maps in the post-game that are specifically designed to get from level 1 to 9999 in two or three battles, when you can start reincarnation and increasing your stats to their maximums.
Also Disgaea 1 has mechanically aged very poorly compared to the recent games. Exp only gained from Kills, Healers can't get exp from healing. Monster units being almost useless compared to humanoids.
So many drastic changes happened in the newer games that makes everything much more enjoyable and oddly enough more balanced compared to Disgaea 1.
How you want to play Disgaea is entirely up to you, but for the sake of efficiency and preventing burnout. Just stick to a small group of your favorite characters and get them as strong as possible to handle everything, then worry about the post-game for grinding.