This guide is for people that doesn't touch most DLCs, especially those Additional Content DLCs. And of course, you want to get the 100 million Credits achievement.

NOTE: As long as your CREDITS HELD are 100mil+, YOU ARE QUALIFIED, not current Credits. To check Credits Held, go to the Menu, then Nepedia, then Play Data, it's there. So spend your bloody credits as you please and dont worry a single thing.

Credits UP, which is crafted from a Plan, which is a Hidden Treasure in MS Mountain Peak. Then to craft it, you need Time Bangle, which is crafted from a Plan, which came from a Chapter 3 Chirper Event. It's hard to get it, but worths. Trust me.
Iffy's Qatar, which is a reward from Colosseum. You need to summon all the Sisters first, then craft the bonus Colosseum fights.
Discs with Red (button?) Level 5 and preferably high level Yellow. Gives the Red one "War" - Credits Up lv5, and Yellow anything that increases Critical Rate, preferably "Baseball' - lv5 critical rate.

You only need 2 girls for this , which is Neptune / Noire (or high leveled Plutia if you bought her) and IF. The point of this is to let the first girl gets enough hits to gain EXE, then use IF's EXE Drive to end them at once, before the Polytan has any chance to fight back.

Equip Neptune with any weapons that has "Slash" attack (Claymore, Esperance,Demon Blade Curse,...), which makes her attack very wide, enough for 3 Polytans to be hit if positioned correctly. Then equip Sharicite (Chapter 7 Overlord Momus drop) to her. In short, focus in increasing her AGI and TEC as much as possible, so she can attack first and do not miss any hit. Then to her combo, use 2 High Rush and 1 Nep Slash in the Rush category, and remove all of her EX Finish, it wastes more time performing all those.
If you're using high leveled Plutia (high enough to out-AGI IF), then switch Sharicite for Move Memory, so you no longer need to HDD ON at the start, wasting way less time.

Then move on to IF, equip Iffy's Qatar for the most insane damage, Histy's Bookmark, and standard Cellphone Strap. Anyway, try to increase her damage as much as you can, high TEC as well, but AGI between the first girl and ~800, so she can attack right after the first girl, before the Polytan has any chance to attack. Also equip the Disc mentioned above to her.
IF's EXE Drive also has one of the shortest time to perform, making her very ideal to use.

The first girl with wide attack hit 3 polytans to get 50 combos. If her move speed is low, then she might need to HDD ON to reach them. Then IF go in and use EXE Drive to kill them all. Note that her EXE must be aimed carefully at the middle Polytans to hit all of them, or yeah, you wasted time killing 2 and left 2 alive.
Repeat for like 30 times, take a break, go to shop, sells those newly acquired items , save game, repeat.



That's it, hope you waste as little time as possible doing this nightmare. Good luck.

Credits to Seb for this idea.

In this game, you can sell your stuffs for 20% the price of items, or 50% price of the Accessories or Costumes, and this will still count as the credits you gained. So in order to massively reduce the grind time, when you're at least 54 millions, or preferably 58 millions, buy all the stuffs in the Accessories and Costumes shop, then sell them. REPEAT until you get the achievement.

Explanation:
Since you get back 50% of what you spent, for example you use 50 mils to buy then sell, you will get back 25 mils to rebuy, we can use this Math formula to calculate it: X * (1 + 1/2 +... + 1/2Y) , with Y is 2 to infinite plus.
So if you have 54 mils, it becomes: 54 * (1 + 0.5 + 0.25 + 0.125) = 101.25 mil, so after selling 3 times, you should get the achievement.
In a better case, having 58 mils only requires 2 loops of selling: 58 * (1 + 0.5 + 0.25) = 101.5 mil.
Best way which requires selling only once is at 67 mils, but needs harder grind so it's not recommended.
Please note as it has diminishing returns, having one to two less mil (52-53) requires another loop of selling since 53 * (1 + 0.5 + 0.25 + 0.125) = 97.5, and another additional loop if you only have 51. And it's practically IMPOSSIBLE for 50 mils as the total of (1 + 1/2 + .... 1/2Y) will always be less than 2 (and only equal to 2 when Y is at infinite, which is impossible).