JUMP FORCE

Now, while most people would just go and farm 'Go for the Gold 1' on Expert since it seemingly grants you the most gold with 5,900 gold per run. There are actually levels that grant more gold, but they take a while to unlock.

First off all, grind to level 100 using 'Level Training 1'. It should take you about 50 times to hit 100 and you'll still get 2,600 gold per run so it's not a waste.

Now, when your character (I think it could be a support character too but I used my CaC) reaches level 100 you unlock a new subset of missions in Expert called 'Defying Limits.' There are 14 of these missions and they provide you with increasing amounts of exp and gold (S-rank rewards are generic clothes you can buy).

'Defying Limits 1' starts with 4,500 gold and it ends with 'Defying Limits 14' which gives you 24,000 gold.

That's not a typo, 24,000 gold per run.

So, assuming that you have some starting money (300,000 since this is Expert after all) It would take you 119 runs of Going for Gold 1 to get the achievement.

This way, even if you have to do Level Training 1 50 times to hit level 100 (still getting 2,600 gold per run for 100,000) it would take you 30 runs to get over 700,000 gold.

This is not even taking into account that 'Defying Limits 1' - 'Defying Limits 13' grant you a total of 175,000 gold.

There is probably some number that would make 'Going for Gold 1' the faster method, but you can do 'Level Training 1' around 80 times before you get near the 119 runs in my example.

I will also mention that using Battle Style C I never needed to do Level Training 1 more than 3 tries to level up. I believe 99 to 100 just needed around 72,000 exp. I'm mentioning this because I don't know if Battle Style effects how much exp you need to level up.



Edit: Added screenshot of the Defying Limits 14 mission and the rewards. If you can't see the image the Rewards are: 85,000 Experience, 24,000 gold, and Mask 03 (Ichigo's Hollow Mask).

I found the expert free mission go for gold 1 to be the best for grinding out gold. I did contact PhantomSiren (one of the first people to get the full completion of this game), and they recommended the very same mission. It will give you 5900 gold each time. I needed 645k after doing every key, extra, and free mission. I even did the multiplayer achievements before this, and didn't break 400k. Be prepared for a fairly long and tedious grind.
Also, you don't need to have the 1m on you at once, you can spend it.