Best thing you can do is breed for the best nature that you can get as well as at least "quite impressive in ability" according to the IV judge. You will go mental if you try to get perfect stats in everything. Even getting one perfect stat is impressive. Plus, it takes a lot of time to breed anyways. In Emerald, if your female Pokemon is holding an Everstone, its child has a 50% chance to have the same nature. This does not work with male Pokemon. Male Pokemon pass down certain egg moves. Some will be worthwhile to get, like Aromatherapy Chansey.
You should EV train, the best thing to do is to get a Pokemon with +3 EVs in a stat and remove all of their moves except for something useless, then go to the Desert Underpass and have a Ditto transform into that Pokemon. Then just take out the desired Pokemon you want to EV train and do that. Also use Vitamin items as much as possible to get at least 100 EVs out of the way. This cuts down the time significantly. Also, make sure you have the Macho Brace. To count how many Pokemon you've taken out, see how many you need to defeat first to max your EVs, then count THAT number instead of total EVs. Use either an online counter (if such a thing exists) or a calculator with ans = ans + 1 so you increment 1 every Pokemon defeated if you are concerned you will lose count.
If you saved after your mom talked to you right after the Pokemon League win, congratulations, you cannot soft reset for Latios. If you're emulating, just get an Eon Ticket. You can still get an Eon Ticket through flashcarts, but if you're doing that, you may as well cheat.
Use all exploits that you feel comfortable doing. Seriously, duplicating Rare Candies isn't much of a cheat in my opinion saying that the AI will often cheese your team. I cloned Rare Candies, Vitamins, and a few Master Balls to easily catch legendaries in order to soft reset faster.
