Welcome to Rayquaza Forums appudds! To add on to Chris' comment on Battling Simulators, mainly Shoddy, I'll try to help you.
As far as I know, I'm the only one on this website who regularly shoddies. I upload shoddy battle vids to youtube (if you would like to see it, please visit the Videos section and find my thread about the youtube links).
Shoddy is not the only battle simulators. There are others such as Pokemon Online and NetBattle, both of which I am unfamiliar with. However, shoddy is a fairly simple tool to use. You start by going to TeamBuilder and selecting Build Team. Click the dropbox to choose which pokemon you want to use, fill in the EVs however you want them, select the Item, nature, and ability, and select the four moves for the pokemon. Rinse and repeat for the other 5 pokemon in your party. Once you do that, save your team into a location and a name you will remember. Then click on Load Team on the Find Tab.
On most shoddy servers, there are tiers such as OU, UU, NU, and Ubers. Their rules correspond to whatever those rules are for wifi. The two biggest things that make a difference in shoddy compared to wifi is the accessibility for using the Rotom forms (who have considerably better defenses than the regular Rotom) and the lack of Acid Rain (a glitch in platinum when someone uses pursuit on a fleeing pokemon and ohko's in sandstorm. Depends on what cartridge is used to host the battle). Some minor glitches involve a Grass Knot glitch (slightly less damage is dealt), the lack of the U-turn glitch in D/P, and the multi-hit-move glitch on shoddy (When a multi-hit move activates a focus sash on a pokemon, the said pokemon dies on shoddy. Supposedly, the pokemon should not die in the actual game. I haven't tested this on Wi-Fi, I'd greatly appreciate it if someone could test that out or show proof of it.)
There are also two methods of getting battles. There is the simple challenge in which you go to the Users Tab, right-click on someone's name and hit on Challenge. There is also the Ladder system (also known as the Find Tab) in which a ladder ranking system is used to determine how "good" you are. If you win, your rating increases; if you lose, your rating decreases. However, suppose if you have a high rating and your opponent has a low rating. If you lose, your rating goes down a lot and the opponent's rating significantly increases. However, if you win, you won't really get a lot of rating, and your opponent won't really lose much. Disadvantages of the Users Tab is that the matches are Unrated, as well as Tiers won't be enforced, meaning that Latios could be used in a match that says NU. The main disadvantage of the ladder tab is that you cannot challenge someone to a rated match, your opponent is chosen randomly.