Fairly self-explanatory - earn 10,000,000 G from playing the various game modes. You earn gold from winning battles, and can get time bonuses and special bonuses (e.g. for perfect victories).
It's worth noting that some modes will give you a big bonus (2,000,000 G) the first time you beat them - e.g. arcade mode, ghost battle, team battle and time attack, so it's worth checking the various different modes out. Other than that, I found ghost battle the quickest and most efficient way of getting this. When you have the option to pick your next fight, go for the option surrounded by a gold rectangle if available. The random bonus for winning this fight can also include gold.