If, like me, you can't get a 2nd controller to work here's what you need to do to make this achievement easy*
All 300,000 points must be earned on 1 credit. If you game over and continue, your score resets to 0. To get all the points you need, abuse the game's save state system.
Its a little confusing. So here's how you do that:
setting the save state:At any time (I suggest doing this after each win), press
and select the first option to set the save state.
restoring the save state (please correct me there's an easier way to do this)
press
hilight ACA NEOGEO SAMURAI SHOWDOWN III and press
hilight 'quit' and press
again.
When restarting the game and selecting whichever version you were playing, it will load up the point you made the save instead of the usual NEO GEO boot screen.
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Tips on how to play vs the AI
The Gaira jumping
strategy is solid and works, but far less effective than on the caravan and high score modes. For some reason, the AI in SS3's original mode is VERY stingy on giving up perfect wins. Its so unforgiving and unfair to the point changing the game's difficulty doesn't appear to make it any easier. I don't know how many times I've seen Rimururu go beast mode and kick my ass in as soon as her family comes out of the house on her stage. I don't remember her being like that in the other modes!
Seriously, the winning strategy for getting this achievement is to rely on save scumming the save state feature.
(*actually not easy the AI cheats)