No cheating, of course, and no trading or events. I already beat the champion, if that matters.
I asked this on Smogon as well, and they gave a better answer than the Sadistic person did.
As for where to get the money, there's a Rich Boy that can be rebattled just outside the south entrance to Petalburg Woods, who only has a Linoone and pays out p7,200 from the fourth rematch onwards. The Linoone holds a Nugget too, which can be stolen if you have Thief. There's also a Lady with the exact same properties and payout north of the forest. Petalburg Woods can be traversed north-to-south without stepping into grass, so you can fly to Rustboro City, go back-and-forth to Route 104 until both trainers want to rebattle (check the Trainer's Eyes), go down and rebattle them, and then fly back up and repeat. Bring a Pokémon with Thief and an Amulet Coin, and you can make a cool p38,800 per cycle.
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Assuming you already have at least P240,000 in cash (and a league run is guaranteed to provide 32,400 of that, more if you were using an Amulet Coin), the most hassle-free way is simply buying the coins.
