Short Summary: Get Secret Base. Upgrade to level 2, and buy Devon's upgrade. Get a Gastly/Haunter/Gengar with Curse, Mean Eye and Hypnosis (I advise you to grind some so it's level 25+ and evolve, Gengar can be obtained by using the PokeTech designer's upgrade), a Gible/Gabite to train, 20+ of potions, super potions and revives (buy at store or mine revives). Also have some fodder pokemon to buy time, 2-3 should be enough, or get a pokemon with Sturdy (Pineco). Save before battling Devon / right after defeating Devon. Battle Devon multiple times, format is as follows. Follow more or less exactly.
Lead with the Gible/Gabite (hold Lucky Egg) before switching to Gengar (hold Amulet Coin). Gengar uses mean eye then curse on Machamp. Don't heal up Gengar. If you can make Machamp sleep before he dies, throw Gabite back in for more xp.
Devon should try to use Conkeldurr. Gengar should be sent out if not already in. Try to curse the damn hulk; either Gengar outspeeds, or pray that Gonkeldurr misses Stone Edge. If Gengar's KO'd before it can land Curse,
ragequit reload the game and try again. If Gengar lands curse, try to stall as much as you can - Hypnosis (if Gengar's still alive somehow, but don't push your luck), or switch into some poor sucker pokemon. Sturdy's useful in these cases. While the Donkelhurr's dying, revive Gengar. Old man Konk will eventually kick the bucket.Grab yourself a beer and drink to the glorious and satisfying death of the ritual sacrifice.
Then go into this format. Switch back in Gabite. Then switch back to Gengar, mean eye, curse, and heal up. Occasionally, Devon will swap pokemon as you try to mean eye them. Either try to salvage the exp (figure this out on your own), or play safe and kill it anyways.
Rinse and repeat, and before you know it, you've beaten a team of level 50's with 2 level 20ish pokemon as well as your fodder. Save immediately, do not talk to Devon after victory - you'll just rebattle again. You get loads of xp and cash. Use cash to restock when neccessary.Use the bed to heal up. Then save, and battle Devon again. Keep rinsing and repeating.
Eventually, your Gabite will evolve into a Garchomp. Train it some more, and occasionally pit it against the Machamp. If it can solo Machamp and win, you train faster.
Then the fated day comes, when your Garchomp can sweep Devon's entire team by itself. Spam Dragon Claw and maybe Earthquake if you have the TM for it. Garchomp should be able to solo at level 53-ish (specifically, it defeats some pokemon, levels midbattle, and grows strong enough to battle the others). Feel free to use Gengar when needed; though at that point revives probably aren't needed.
Results: level 60+ Garchomp, level 50+ Gengar (training Gengar to high levels not neccessary), and a few hundred thousand dollars to spend on whatever you want.
I advise you train Garchomp to level 80 or higher, which is about when it can oneshot every pokemon on Devon's team with Dragon Claw.
Short Summary ends here, continue down for reasons to have excessive bragging rights.
Thus, you walk into your secret base with a few ten thousands of cash, a Gabite, a Gastly, and a lot of determination to find and expoit gimmicks for lol's (for me anyways), and walk out with a pimped out level 80+ Garchomp with (near) maxed Attack EV's, a level 50+ Gengar for backup, and several hundred thousand dollars.
Oh yeah, Agassiz Town's gym pokemon aren't even level 30. Also, Caspian City's gym has Miranda, the final Gym before the elite 4. Her pokemon go up to level 70.
I did a quick peek at the champion of the region because I haven't reached that area yet; so I looked at the pokemon levels mostly (I have a bad memory, so no real worries of spoiling myself). The champion's pokemon go up to 79 (I think). Remind me, exactly how high did I advise you to train Garchomp?
Oh, right. LEVEL 80. And remember, you can repeat this process as many times as you like. Feel free to have a team of 6 LEVEL 80 GARCHOMPS. Even better, take it to the extreme, and train them to level 90 or 100 just for fun.
Congradulations, you've basically broken the game by reaching lategame potential (read: level 80+ pokemon) before you've EVEN BEATEN THE THIRD GYM.
Long Version (incomplete)
This involves using the Secret Base, obviously.
Usage of this trick requires a Ghost type pokemon that can use Curse. Preferably, said Ghost pokemon can outspeed a level 57 Conkeldurr to make things more consistent (instead of praying for the 20% miss chance of Stone Edge. I highly recommend using a Gastly, because it learns Curse around lvl 20 ish and also learns Mean Eye, which will greatly lessen the usage of Curse (which will save you money).
I advise you get a 'main' pokemon for
obliterating all the puny trainers, gyms and misfits in the world that stand in your way battling. I got a Gible early on. You can get one too in Midnight Way. Besides being a pokemon that eventually evolves into a beast that slaughters all pokemon in its path the pseudo-legendary pokemon Garchomp, it also has early access to Dragon Rage, which does 40 damage. There's no Fairy pokemon in the game (at this point anyways), and thus it's a OHKO early on. You can grind for Speed EV's at Route 306.
Why Pseudo-Legendary pokemon? 600 Base Stat, which makes them as good / better than some legendary pokemen (lel). You gotta have a way to counter those scrubs that rely on Master Balls to catch the good stuff. Those pokemon have the title "Pseudo-Legendary" for a VERY good reason.
First, you gotta meet some requirements.
1) Reach Agassiz Town, for obvious reasons.
2) Have or save up lots of cash to buy up all the upgrades for the Secret Base level 1. Then upgrade to level 2 via that thing that upgrades your base permanently.
When I got to the town, I had a massive amount of cash, due to me only spending money on pokeballs, as well as using an amulet coin and defeating all the trainers in my way. So basically, at that point, I could've completely upgraded the base to full level 2. DON'T DO THAT, IT CAN AND WILL RUIN THE TRICK. (or at least make it more time consuming.)
3) Buy Devon's upgrade, so you can battle him over and over.
4) Have a Ghost pokemon that can use curse. See above for more info.
5) Optional: have a pokemon that you want to use as a main, preferably a physical attacker.
For me, I had a Gabite. It had a Jolly Nature, not perfect IV's (later corrected as a Garchomp w. IV stones gotten from god knows where (mining cave? IDK)). As for its EV's, I earlier grinded at route 306 to max out the speed EV's. Took a long time, but it's 100% worth it (in the long run, anyways). As a bonus for grinding, it was 10 levels higher than the wild pokemon in the area, and easily cleared the way to Agassiz Town.
6) Have some Revives (10 ish should be enough, maybe get 20 to be on the safe side), as well as some healing items. Potions and Super Potions should be enough, maybe 20 ish of each. This is NOT mandatory (technically), but neccessary if you want to MAKE money early on instead of losing money. Also keeps your pokemon a little more happy, and makes EXP gain MUCH faster.
Now, here's the drill for EASY MONEY (BIZZARO FLAME) as well as easy xp.
What we're going to do is expoit Devon's weakness. Namely, all of his pokemon have normal or fighting type moves, bar Conkeldurr's Stone Edge. Therefore, any ghost pokemon will be 100% immune to his team, with the previously mentioned exclusion. However, the pokemon have high defenses too (with 2 full restores total to help), hence Curse (which does 1/4 total heath for damage per turn at an initial cost of 1/2 your pokemon's HP, rounded down) and the potions.