A Repel is an item which lets the player walk a certain number of steps without encountering wild Pokemon with a lower level than the first member in the team.
Repel Trick is using repel, choosing the appropriate level to only encounter the pokemon you want to hunt, or at least, a lower amount of the pokemon you don't want to encounter.
Well, you will need tons of repel then.
Repels can be bought in almost every pokemart.
There are 3 Repels, the only difference is how long it will be effective.
You also can get repels by using Pick Up, on every pokemon in the game.
Once you have your repels, you need a "repel trick slave".
The first thing you want to know is what level it has to be. Most of the time, taking the lowest level possible for the specie you hunt is the best choice.
Then, you can pimp your slave by having a cool ability on it.
Some cool abilities are :
-Syncro (using the nature you want the most)
-Illuminate (To encounter mons more frequently)
-Arena Trap (To encounter mons more frequently) <- Maybe swarm works aswell for it ? To be confirmed
-Static (If you're hunting elec-type pokemon)
-Magnet pull (same but steel-type)
Here I will list some repel tricks, and perfect repel tricks.
Perfect repel tricks make you encounter only one specie, others just lower the non-interesting species amount.
Perfect repel tricks
Other repel tricks
[I will edit to add a lot more in the list, feel free to contribute]