To earn 10,000,000G -
This trophy seems a grind but it can be obtained very easily.
Winning a fight can net you roughly 15,000G - 50,000G each time depending on how you did.
So If you was to win 25,000G each fight this would take 400 fights!.
However there are shortcuts to the 10,000,000G for example -
1 - Finish Arcade Battle for the first time will net you 2,000,000G
2 - Finishing your first Team Battle will net you 2,000,000G
3 - Finish Time Attack for the first time will net you 2,000,000G
Thats already 6,000,000G now only 4,000,000G to go!
You could choose to knock the rest of the cash out in Ghost Battle which with some fights Win or Loose you can obtain a random lucky box which will sometimes have G inside of it (The least I got from a lucky box was 15,000G and the most 600,000G), Or head into Fight Lab and play Stage 4 / Boss Battle and do the best you can on it for very easy "G", Doing good on the Boss battle can net you 100,000G+ each time and can be done within 50secs to 1min 30secs Depending on skill.
Hope this has helped!.