Valkyria Revolution

Having messed this up myself, I thought I would explain what you should do to save yourself some time on this pain train. First off, don't spend much on any secondary weapons throughout your whole playthrough (I would primarily upgrade the core weapons of your team, for me being: the standard machine gun, the standard grenade, and maybe the standard RPG for tanks); essentially, you are saving for some save file scumming later down the line, to save you 5+ hours of extra grinding I had to endure. Otherwise, feel free to buy minimal stuff from shops around town, aside from crafting 30 armors for
and lvl 1 ragnites for early game power (buying these do count to your 300k).

So, once you reach ~320,000 credits (which can be best farmed from Chapter 10-1 in postgame and NG+ with Sara's magic or whatever your strat is), save, and you now have a file to buy all the secondary upgrades to get
Then reload that save and buy the most expensive workshop item at your disposal to spend your 300k.

As a final tip, if you messed up like I did (and probably most players), you likely spent all your hard earned 10-1 money on secondary weapons, thinking it would count towards the 300k, meaning you will have to grind more; it sucks. However, there is a trick I used which does work and will save you 25% of your time. This trick being the exploitation of the Tannery economy to buy-sell efficiently with the top three cloth options. So, assuming you have not spent anything on the shops, you will need ~240,000 credits and a hour of mindless dpading, or so. What to do is buy the first three cloths at 12c each maxing all three at x99. Then back to 'Sell' and sell all of it back, effectively spending 75% of your money (buy at 12c, sell at 3c). With this trick you should be able to get this slightly faster then farming 10-1, 33 more times (assuming you won't need more ragnite, which is a whole other discussion with
With all that, best of luck.