Winning Double Battles
Double battles are a significant part of Pokémon Emerald. These battles challenge players to use more than one Pokémon in their lineup against other trainers. It's a time-consuming aspect of the game, but it is helpful in the long run. Don't avoid double battles--use them as a tool to train your Pokémon. Level up a weak Pokémon by sending them out with a stronger member of your party to gain experience points. Double battles help you level up faster and encourage your party to grow more quickly than previous versions of Pokémon allowed with single battles.
Fighting Gym Leaders
The gym leaders in Pokémon Emerald are tougher than ever, which is why it's important to train vigorously before attempting to win a badge. Battle all the trainers inside the gym before approaching the leader. This will give you an idea of what type of Pokémon the gym leader will have, and you can better prepare your lineup. Trainers in gyms typically carry more cash on them as well, so beating them is more than just a tactic to gain experience points.
Stock Up
You can't afford to be frugal in Pokémon Emerald if you are making a serious attempt at beating the game. Because of the double battles, difficult gyms and villains, it's crucial to have a well-stocked inventory at all times. Carry at least a dozen potions on you and a handful of Poké Balls while trekking across the region. Full Heals are best to carry, as opposed to a number of different status healers. However, you may not be able to find these items until later in the game. Stock up on at least a handful of Antidotes, Awakenings, Revives and Paralyz Heals to play it safe. You can't invest your money in the game, so you might as well spend it on a good cause.