First of all you will need to play in Normal mode, second, you'll need to buy the item Reeking Box which costs 500 gold. Use the reeking box in the overview map and a fixed encounter enemy unit will show up.
Enter the battle and one of the enemy units will carry at least one bullion which you can kill the enemy for. After you clear the map, you can sell the Bullion's for 1000 gold a piece and you should earn money in this game as well as level up your units.
