You don't need any sort of cheat code to do this. All you need is patience, and a fighter at level 32 with long-distance attacks (I prefer to use Hiei and his Dragon summon). This hint is for people who have beaten the game, but still have some fighters to level up.

At the last area of the first game map, go to the place just below the right-most gray spot. It's the Pursuit level.
Your fighting team should include Hiei in the first slot (or a level 32 fighter with distance attacks), and any fighters whose levels can still be raised.

Right away, the Runner demon should move a small distance away from you. Your level 32 fighter should move next (Hiei does, at least). Go up the steps to get closer to Runner, and use Hiei's Dragon Summon. It should instantly kill Runner, and each fighter below level 32 that was in your team will get almost 700 experience points without even attacking anyone. Keep doing this until you've got the levels you want.

To take on a big damage on your opponent, you need to use overdrive in the character's best moves like spirit wave, yoko death plant, and dragon summon.

In the level where you get Chu don't use Kuwabara anymore while fighting monsters because he is slow, and the highest level you can get in this game is level 20.

P.S. I suggest you use Yusuke, Kurama, Hiei, Masked Fighter, Chu while fighting monsters.