The best Farming method for this can take some time to set up. You need to beat the final Arc on Hard. Once that is completed, you can load up Chapter 8 map 14. What you want to do is clear the way to Team Yamcha and Team Piccolo. Every turn there is a chance for an enemy to increase in level.
You want to increase the level of both teams to their limit. I believe Team Yamcha can go up to LVL 91 and Piccolo to LVL 94. If you dont get the level up on the correct team, return to story mode title screen and then hit Continue on Hard mode to restart from the last time you saved. So every time one of the teams levels up, you will want to save the game. Enemies do level up after fights as well as there will be 2-3 teams you will need to defeat that are in your way. You have 22 turns to level them up 7-9 LVLs.
At any point when you get next to team Piccolo or Yamcha, switch your team to some expendable guys you dont like and initiate battle. The goal here is to weaken all three fighters and then lose. That way when you got those teams leveled up you can end the fight quickly.
Once you got the teams to level 91 or higher, save your game and then win the 2 fights using 3 LVL 6 Zeni Bonus skills and you will rack up 6k+ zeni in just a couple mins. Once you defeat the two teams return to story title screen and reload your last saved game for Hard Mode.
I take no credit for this solution. A youtube video below is where I found this all out