Play mission 5 with Kintoki. Make sure you have his second or third axe. Try to kill as many Stone Golems as possible at the end of the level. You will be guaranteed at least 2,000 gold pieces each time you finish the level.
Note: You must be up to the stage where you fight both centipedes. Use Tsuna and his second weapon, Thunder, and follow these directions. When you first start the stage, lock on to the red centipede. Try to stay on the center ring through the entire fight. Wait about three to four seconds and he should swoop down towards you from the circular path that he flies in. When he gets about halfway towards you, double jump straight at him and do Tsuna's heavy attack. This should take him down to about half health, and an intermission sequence of the other centipede coming out will begin. You must now worry about both centipedes shooting electric balls at you -- be careful. Also, watch out for the red centipede's electric shower (a little flash of electricity around you). When you see this, jump to your far left or right while dodging the electric balls. Keep doing the same thing to the red centipede (waiting for it to swoop down at you, then doing the power attack). This should kill him. When he dies, the female centipede will say "My Liege, nooooooo!". When this happens, try to lock onto her. Do not worry about the electric balls and the showers, as they will stop. If she flies around for a little while then disappears from the screen, wait until you see the tip of her feelers come into view from the top of your screen and press Y . If she flies around for a moment before coming down low and snaking across the ground, wait for her to get close and press Y . If she flies around for a while then comes down low and dives under the water, get directly in her line of sight and double jump as she pops back out of the water to leap over you, and press Y . You will hit her as she jumps over you. If you defeat both centipedes in enough time, you should get about 25,000 gold and 60,000 experience points.