Pokémon

The trick is to plan from the start by keeping some zigzagoons in your team for their pickups. They will sometimes grab rare candies as well as potions quite often, and they also are good HM... helpers. If you don't like to grind, you can "upgrade" them around Fourtree City, where they spawn at higher levels, improving the pickup items. Hold every rare candy you get until you reach the league and your pokes are leveled, so they grant more experience. Ik you are already past the point of this being helpful for your current run, but this is for the future or anyone else reading.

I usually just spend all of my money and farm directly the elite fours.

You go in, kill some pokemons and die. Rinse and repeat.

There’s a loop you can do fighting the interviewer and cameraman. It takes a few times beating them before their team levels up, but it’s relatively quick exp due to it being two higher-level, evolved mons in a double battle.

If you put exp share on a mon that is out in the battle, it gets a higher share of the battle xp so long as it doesn’t faint. I did this to level most of my mons for the living dex.

This is all vague as it's been years since I played, but grinding is pretty much your main way. There's no tricks or minigames to max out EVs, IVs, etc at least until postgame when you unlock the Battle Frontier. They didn't start heavily leaning into those QoL features for another few gens.

Don't be shy about using legendaries or your starter (or other "OP" Pokemon), and maybe try to build your team for type matchups. It was also assumed by the design that you'd be using healing items and revives at least between battles.

Some HMs are legitimately good moves. If you have money to spare there are also a good amount of powerful TMs you can buy in various places and move tutors.

Also, there are some mons that just suck due to low BST.

This is from before they nerfed difficulty down to be beatable by anyone who picked the game up. It's not kaizo by any means, but you can't just mash A either.

Good luck.

When I was playing it I found that preparing for the later fights to be better than the first 2.

Surf the seas to find any straggler trainers and use EXP share on Victory road and while you grind the first few trainers in the elite 4.

Plus if you have the cash (or the amulet coin) spam health items until you win.

Being good enough is a virtue

Gen III is so damn low level/exp everywhere.

There's no great way to grind levels. Anytime I play gen III (often in a multi world randomizer) I bump exp up so I can use more than 1-2 pokemon without grinding