You must have Hiei's full stats for this to work. Go to the level above the first level in the Dark Arena. Have Hiei in your party first. Select any other characters you want to gain experience and start. Have Hiei move to the space next to (on the left) the light pole and choose Wait. Then Move 7 spaces upward (using the left button) and wait again. Then move to the red square and wait one final time. The battle will be over and all of your characters will gain +650 experience without even giving the enemies a chance to attack. Repeat as many times as desired for effortless experience.
Submitted by Hunter McCullough