First, you must have the vacuum arm maxed out in all of it's statistics. It's also nice to have high attack and range. Then, go to the Cardon Ruins, and look in the room right next to the refractor. Shoot the bee hive looking things, and use the vacuum arm to suck up the zenny really fast. Also, Shoot the bugs and the robots for extra zenny. It should all equal up to about 5000-7000 Zenny. Then, don't exit the building, but right next to the exit is a ledge. Jump up and go through the door. Then, turn right around and go back to the room with the bee hives. All the enemies will be back! Just repeat this until you have enough money to satisfy you.