Managed to test this thoroughly with about 85% win rate. Ignore ranks, forget matchups; here's how to get through this minigame:



Step1: Check the statuses of all the fighters. You can see this at the bottom of the screen when you can pick your fighter.
Step2: See if Maria's Status is "Cleaning her gun". She will be our winning gal.
Step3: If so, check if her opponents have relatively bad statuses. Tired, annoyed, etc. If not, go out and re-enter to reshuffle their statuses.
Step4: All good? Pick Maria, and ALWAYS GO FOR ROCK! Seems weird, weakest fighter with weakest move, but trust me it works.
Step5: When you use this strategy, opponents will always go for just 2 of their moves repeatedly, with one favourite. only switch off rock if they use paper 3 times in a row. If they switch after you do, switch back to rock. Rinse and repeat. You'll lose some rounds, but should come out on top at the end of every fight.

!ADDITIONAL INFO, IMPORTANT!
- When you both pick the same move and you need to mash. DO NOT MASH AS FAST AS YOU CAN. Start slower, and gradually move up to as fast as you can go. This will not work 100% of the time, but it's far more consistent and will give you the edge you need.
- Do NOT quit when you're about to lose with Maria! Her mechanic is that when she goes down, she has a chance to take down the opponent afterwards instead. The chance that she does this is really high when she has the "cleaning her gun" status, hence this strategy working so well.
- Do NOT skip your fighter's rainbow moves as you can mash to increase damage.

I've noticed this strat works with Francisca as well IF her status is good and those of the others are bad. She has the same comeback mechanic. Picking what move she has to do is a bit more RNG though, so I still recommend Maria. Consider her a second pick when you spot everyone else is doing bad. Maria's matchup against Sayuri still isn't great, she also spams rock a lot. Check if she's got a bad status if she's likely to show up.

This has worked for me really well so I hope I can help some of you guys out with this strat. It's a bit tedious waiting for the right statuses, and the fights can take a while, but it's far better then going by RNG. I recommend putting the game on low volume and starting up your favourite podcast to mitigate the time investment somewhat.

Thanks for checking this guide and let me know how it works out for you guys!