One Piece: Burning Blood

You need 2 controllers for this method to work. You will get 300 000K per matches.

1) Two controllers
2) Go to Pirate Base -> Create Pirate Crew -> Make a crew with Bartholonew Kuma only and save (You press Start and A then B)
3) Free Battle
4) 2 Play
5) Press Y to Call Pirates and select the crew with Kuma only
6) Same for Player 2
7) As soon as the match start Player 1 press LB + Joystick Left DOWN + Y and hold them. There should be a Pink/Purple Aura around him and he will wipe the other one out of the surface of the planet.
8) Look at the results and fight again.

If you haven't unlock Kuma you need to be done with the story first I'm guessing. All credits goes to YouTube and my mother for putting me on this earth.

The above solution is good and very effective on how to earn the 100 mil beli. However I found a quicker way to obtain the money and hardly any work required.

Along with Story and Wanted missions there is Pirate Flag Battle. You choose a faction and go to war in attempts to make your faction the best of them all. It goes on a time period like other games do for competitive ranked modes where it'll reset after so many days. I joined one and a few days later checked on it and the first period ended and I was awarded 30,000,000 for participating in the war. However, I never did a battle after joining. So once you completed all other achievements and don't feel like grinding you can do this and will count towards the 100 needed. Not only does it save you time but requires less effort than grinding one wanted poster over and over again.

The only downside to this is time obviously and wouldn't fair well with people who are renting it or received it from gamefly. But if you bought it then this solution will be right up your ally.

Edit: Join one of the following factions to make it worthwhile as the others don't regulate anyone joining basically giving you nothing. These three will always give you the most bang for your buck. The others are pathetic in terms of benefitting you even if you like Ther other factions sadly.
1. Revolutionary Army
2. Straw Haw Pirates
3. Whitebeard Pirates

This will probably be your last achievement and i recommend that you save it for last as you will be gaining Beli(The currency to buy battle and support characters) throughout the story and WANTED list. Even when i had every achievement besides this one, i still needed about 20 million beli.

If you have gained all of the secret achievements, then you have unlocked all of the WANTED List challenges. You should use to the very last one, A Queen's Request, and play either the tenth or the eleventh poster. The battle is against nine opponents and if you win, you receive 1,200,000 beli. I found this to be the fastest way to grind beli and it still took me a couple of hours. Try to use characters you are good with because between rounds you do not regain health.

The fastest method to earn Beli is to set up a local game with 2 controllers.

Player 1: Bartholomeo Kuma
Player 2: e.g Usopp (doesn‘t matter)

Remove the other possible character spots.

Select any map, I prefer the one with the green island, and start the match.

Player 1 hit + to teleport Player 2 away and the match is over.

This takes around 4 seconds and you will earn ~300.000 Beli.

Now select repeat match and do this over and over again until the achievement unlocks.

Note: This only works for earning Beli, not XP. I suggest to wait with this achievement as your last one.