Like other Pokémon games, the battles in Pokémon Brilliant Diamond and Shining Pearl get increasingly more difficult as the player travels across the region. Although trainers battle low-level creatures when they first leave Twinleaf Town at the start, they eventually have to face the toughest characters in the Sinnoh region by the end.
In order to keep up, trainers will need to level up their own Pokémon. Luckily, there are many different ways to increase experience in these remakes, which means that players will always remain on top and ready to battle any monster that crosses their path.
Switch Out Pokémon
Unlike past games, EXP share between Pokémon in the player's party is not optional. This means that, since the beginning of the game, all party Pokémon gain experience from every battle regardless of whether they were used in the battle or not.
Despite this, the trainer should still switch out their Pokémon whenever they can because they gain more experience if they actually participate in the battle instead of just relying on the EXP share. Even if the player doesn't care about leveling up, however, they should still use everyone in their party because it helps raise affection and makes battles with different types easier.
Battle Wild Pokémon
Although random encounters can be annoying and players may try to avoid all grass and cave areas, trainers should try to have at least a few random encounters whenever they enter a new area. Since wild Pokémon are easy to deal with because they are usually alone and don't have access to items, they can give the player some easy experience.
If the player isn't sure about whether their level is high enough, fighting wild Pokémon can be a great way to see if they're still on track since the later routes will have increasingly stronger Pokémon to match where the player should be in the story. Plus, trainers can use these encounters to grab any new powerful Pokémon they don't already have to complete the Pokédex.
Battle All Trainers
While traveling through routes in the Sinnoh region, the player will find random people who will want to battle. For some of these battles, the player will have no choice but to have the Pokémon battle to continue onward, but other times the player can sneak past these people or will have to choose to interact.
But, if the player wants more experience, then they should battle every single random trainer on every route that they can. To do this, the player should talk to every NPC they see. Although some of these battles will happen automatically when the NPC sees the player, other battles can only happen if the player specifically talks to the NPC.
Explore The Region
As the player travels from one major town to the next, it may be tempting to just head straight to the next destination to continue the story and get closer to becoming the next champion. But, throughout the entire region, there are many optional areas and secrets that the player can miss if they don't look around.
At the start, the player cannot access many of these places because they will need certain Hidden Moves they obtain during their journey. Once the player starts getting moves such as Rock Smash, Cut, Surf, and Rock Climb, they will be able to access these hidden areas that sometimes will also have possible battles.
Farm For EXP On Later Routes
Since wild Pokémon in certain routes and areas stay around the same level as they were when the player first encountered them, random encounters in early routes are not very useful for experience grinding. Instead, the player should focus on battling wild Pokémon in routes and areas that the player has only recently unlocked.
This means that right before attempting the next Gym, the player should fight some random battles in the area immediately beforehand. For example, before fighting the Oreburgh Gym, trainers should have some random encounters in Oreburgh Mine, which is the last area the player has to go through before the Gym.
Keep Double Battle Partners in the Party
During the journey, the player will encounter NPCs who need to be escorted out of an area. While they are being shown the way out, this person will be in the player's party and, since this person will team up with the player in Pokémon battles, the player will need to fight in Double Battles. Instead of immediately guiding the NPC to the exit, the player should grind for levels with the person in the party because the player will gain more experience from the Double Battles.
For example, early in the game, the player meets Cheryl in Eterna Forest, and she joins the party because she needs help getting out of the forest. While she is in the party, all wild Pokémon battles and trainer battles are double battles, which leads to more experience. This grinding is also made easier because Cheryl completely heals and restores all Pokémon in the player's party after every battle, which means that the player doesn't need to use any items or visit a Pokémon Center.
Use The Vs. Seeker
When the player enters route 207, they will bump into Professor Rowan's assistant, who will either be Lucas or Dawn depending on who the player character is. Lucas or Dawn will give the Pokémon player the useful item called the "Vs. Seeker," which allows the player to see which trainers would like to battle again.
If the player uses this item, any nearby NPC trainers on the screen that wants to battle again will have two red exclamation points above their head. The player can then battle that NPC again for more experience. But, while this can give the player some good extra experience, the Vs. Seeker does have to be charged after every use by walking 100 steps, which can get tedious.
Visit Optional Dungeons
As the player travels across the region, there are many optional dungeons that the player can easily miss. Even though they don't need to traverse these areas to beat the game, these areas do have some special Pokémon, extra items, and, most importantly, extra opportunities for gaining experience.
One early example of this is Solaceon Town, which the player reaches on their way to Veilstone City. If the player walks to the right off the main road, they can find the Solaceon Ruins. In this small dungeon, the player can find a couple of items and get all variations of the Psychic-type Pokémon Unown, which unlocks the final exit of the ruins. Besides getting these items, the player also gets plenty of experience by battling the Unowns and going through the ruins.
Visit the Pokémon Hideaways
If the trainer still feels under-leveled or if the other options are too tedious, then the player should consider going into the Grand Underground. While in this location, the player can enter Pokémon Hideaways, which are like smaller versions of the biome-specific wild areas in Pokémon Sword and Shield.
Besides being a great way to fill up the Pokédex, the player should also visit the Hideaways often because it is one of the best ways to gain experience. Unlike the wild Pokémon above ground, the creatures in these Hideaways scale their level to match the current level of the player, which means the player will not have to worry about being either over-leveled or under-leveled.
Use Rare Candies
Towards the end of the main story, the player may start especially struggling with gaining experience because it now takes a lot of experience points to reach the next level and the elite four and champion all are higher leveled compared to the eighth Gym. Because of this, the player should save all their Rare Candies for this last stretch of the game.
When a Pokémon is given a Rare Candy, they immediately gain one level regardless of how much more experience the Pokémon needs. Because of this, the player should save any Rare Candies they get until the end when they may need to raise their level by many points.