After winning the End Area 5 Liberation Mission, there will be a gold Mr. Prog there who will recreate the battle. Talk to him and start the battle. Send a Navi to liberate the first item square and you should obtain 1800 Zennys. Next, retreat. You will leave the battle, however you will keep the 1800 Zennys. Repeat as many times as you want for more Zennys.