How to make Pokemon quickly reach the best fighting state in Pokemon BDSP?
How to change the nature of Pokemon? The first thing to do is decide what properties you want your Pokemon to have - usually, it's best to lean towards your Pokemon's highest stats - tank's defense or special defense, slugger's attack or special attack, or attack-dependent Pokemon's speed is number one. As for stats to reduce, it's best to choose an unused attack stat, as taking from anything else is ultimately harmful. This means that if your Pokemon is a physical attacker, lower the special attack stat, and if it is a special attacker, lower the attack stat.
Once you know what nature you want, unlike older games (like the original diamonds and pearls), it's luckily easy to change it. Go to the battle tower, where you will find the shop. The counter on the left sells a variety of useful items to enhance your Pokemon, including natural mints. These are expensive and come in at up to 50BP, but with a little sanding, you can get to the amount you need fast enough. Buy mints that correspond to the properties of your desired Pokemon and use them with your Pokemon.
How to Change Pokemon's Electric Cars
The traditional way to give your Pokemon EV is actually through combat. When a Pokemon knocks down another Pokemon in battle, it gains 1-3 EV for a stat (usually the highest stat of the defeated Pokemon species). This means you can add EV to your Pokemon simply by having wild encounters with the same species and keeping track of how much you've eliminated.
However, it also means that if you've ever used your Pokemon in combat before it's ready for competitive combat, it already has some EV, which is probably not what you want. Not that there is any way to see what they are. Fortunately, EV can be removed, and each of the following berries will remove 10 EV from the corresponding stat:
Hondew Special Attack
Grepa Special Defence
These berries can be purchased from the Battle Tower Shop for 10BP each, and once you have them, they can be grown around Sinnoh like any other berries, which means you can get plenty of berries to make sure your Pokemon doesn't fail EV required.
Once you're sure your Pokemon doesn't have an EV, you can start the above grinding process if you want, but there's a quicker way. Vitamins add 10 EVs to the corresponding attribute and can be purchased at Veilstone Department Store. Buying 51 vitamins is obviously expensive, but fortunately, making money in BDSP is an easy process. You can also buy vitamins for 1BP each at the Battle Tower store, although it may be faster to buy them from department stores.
Here are how vitamins match up to EVs:
Vitamin EVs Given
HP Up HP
Calcium Special Attack
Zinc Special Defence
After sorting out all the EVs, all that's left is the IV.
How to Change a Pokemon's IV
Before you start thinking about changing the IV, you first want to be able to see the IV, because unlike EV, it's actually possible to check a Pokemon's IV. Inside the battle tower, talk to the guy by the wall to the right and he will give you the "Judgement" feature, which you can use on the party or box screen to check the Pokemon's IV. It won't tell you the exact values, but it will let you know how high they are, and crucially, you will know if the IV is 31 because it will say "perfect".
Assuming you don't have five perfect IVs (you don't need to worry about perfect attacks on special attackers or special attacks on physical attackers), you will need to change them. This can be done with super training, which you can access by talking to the person on the right side of the PC between the two Battle Tower store counters. For one Bottle cap, he will use at most one IV. Give him a golden Bottle cap and he'll use up all six. Pokemon must be level 100 to be super-trained.
Bottle caps were the most important currency at the time, and you could buy them from the Battle Tower shop for 25BP each. Gold bottle caps can only be obtained through Masterclass streaks and are indeed a very special reward. There are Pokemon BDSP Held Items for Sale.