G in this game is incredibly useful. You'll need it to buy tickets, buy mugen field level skip items, and most importantly, upgrade your equipment. By the end of your journey to the platinum, you'll need millions, if not billions of G. This trophy only requires you to get about a billion.
The absolute best way to get G is first by upgrading your Damage Carnival. You'll need MP(which you'll get a lot of from the last few levels of the Mugen Field, story battles, or doppelgangers). After that you can upgrade your Damage Carnival in the rest stops in the Mugen Field. If it's grayed out and you can't upgrade it anymore, that means you need to proceed to the next rest stop to upgrade it further. Each time you upgrade your Damage Carnival it increases how much bonus G you earn from hitting enemies into the orbs in Fever Time.
Once you're ready to start grinding you can head into the Mugen Field at any bet. Start Fever Time by Frenzying the big crystal, then do your best to combo enemies with Blast Off attacks into the floating orbs. Time your attacks for when you have high blast off power and multiple characters attacking in a row. The higher the combo, the bigger percentage bonus you'll get at the end. While that alone will net you a lot of G, there is another bonus that the game fails to explain. In the Mugen Field you have a total bonus percentage that can increase over time. Every time you hit an enemy you raise it and a big combo will raise it by a lot. This bonus will continue across your entire run in the Mugen Field. So, if you combo enemies in Fever Time every floor, you'll start to accrue a huge multiplier. By the end of the run, each battle will net you millions. With 8/9 Damage Carnival and getting a combo between 100-300 every floor. I was netting over 100 million G per battle on the final 10 floors. This is far less work than it sounds and can be done on any amount of bet.
With some grinding and the proper use of Fever Time you should hopefully get your trophy and millions of G in no time at all.