Need Help With 2nd Cycle

Sounds like you have a pretty good team. Pretty much, the best way really is to go through the item world for money. However, it is sort of a double sided problem, because if your characters aren't well equipped, they will have problems. Pretty much, there is not a faster way, at least not a safer way, than my recommendation. If you can get through a more simple item world, and raise enough money, use it to buy stronger equipment for one character. After they are all equipped good enough, the item world should not ever be a problem.
Just a little secret, using reincarnation, or transmigration, is also a very good idea. If you take full advantage of it, you could even have a level one character strong enough to take on the final boss alone. Levels are not everything, as each time you reincarnate, you will start off with higher stats.
if you don't believe me, try taking on Baal, the lord of terror. I had used level 9,650 laharl, and nine majins above level 9,000, and they were all taking over 1 million damage with each attack. Seriously...Baal is level 4,000, and he still can one hit kill almost anything even at level 9,000.

First off, the item world is the ultimate place for level grinding and money. However, if you can find some good stages with good exp, you can re-do those stages again and again.
It is recommended that you take advantage of transmigration, as you will have slightly higher stats from each reincarnation.
Along with that, it is very important to recognize your characters' strengths and weaknesses. Skulls and mages are not meant for up close fighting. Warriors are best to stay away from magic users.
It is also very important to get good equipment for your characters. My suggestion is a good weapon, two pieces of armor, and whatever your character needs most (glasses, orbs, etc.).
Items are important to bring in emergency situations. Always try to have HP and SP recovery items handy, and make sure you have a mr. Gency's exit when in the item world (especially in Disgaea 2!!!).
Last of all, it is important to move item world specialists into your weapons and armor, even if it takes a while to do so...fill the weapons and armor with the ones your characters need the most. In my opinion, you should also dedicate this idea to maximizing your resistance towards poison, as even the strongest character is easily crippled by the effects of poisoning.

Hope the info helps