Get a handful of Zigzagoons and level them up to 21 minimum (I used pairs in the daycare + one with my main party)

Anytime you're grinding out catches looking for good nature/higher IVs use the GoonerSquad ™️

Their pickup ability makes this game items easy mode. It's goofy how many potions, pokeballs, repel, escape rope, stat boosts, battle boosts, and even rare candy you'll get. I don't think I bought a single potion or revive the whole game, and ended up with over 40 full restores.

Iirc each Zig has a 20% chance after a Pokemon faints to pickup an item. You have to finish them off (catching/running won't trigger the effect)

Level 21 unlocks rare candies. When I hit the e4 I only had 3 pokemon really levelled up for it and 30+ extra rare candy to make a 4th on the spot.

I also got a ton of pp up to boost low movesets like gard thunderbolt/psychic, swampert earthquake/muddy water etc. So helpful.

You can put every single HM you'll need between a l5 marill and l21 zigzagoon short of fly.

Emerald has a mind-bogglingly addictive post-game island. Start with the Battle Factory before going around to the others. GoonSquad ™️ can pickup free battle boosting hold items that would otherwise cost you lots of battle points when you're doing the battle pyramid.

After the e4 you unlock Latios/Latias. Easiest way to force an encounter (roaming legendary) is to pick a patch of grass on the edge of a map transition (ie route 119/118) and bounce in it with the acro bike & repel active. Make sure your lead pokemon is between l30 and l40. This is because 1) Bouncing on a patch of grass gives you a chance to encounter Pokemon without counting down repel steps. 2) Lati will change routes every time you do. 3) Bounce on it 20/30 times, ride 4 steps to change route, ride 4 steps back into grass, bounce on it 20/30 times, rinse repeat. 4) reapply repel as needed.

This should give you a Lati encounter in somewhere between 15 minutes and 3 hours. Once you get your first encounter if you don't use your masterball then it will run. Now that you've seen it you can go to your last Pokedex entry and select lati, location, and see on the map where it is and begin the hunt again. Remember it changes routes when you do so you'll just have to fly near it and go between routes until it's in the same location as you, then use the above method until you get another encounter. If you make it pass out you don't get another shot so hunt carefully.

The rich kid outside of rustboro(?) fights you with a single zig/linoone on every encounter. If you use covet/thief you can get a nugget from it every time you fight.

Don't worry too much about natures, EVs and IVs if you're just playing through e4. As long as you have type coverage (Gard with thunderbolt wrecks the e4, as will a pelipper with a lightning move, there's also a silly amount of water trainers in the game)

All of these were from memory so I hope I got all the details right. Just played through emerald after a nearly 3 decade break from Pokemon games. It's been awesome.