In Pokemon Emerald version you can get to a place called the battle frontier. There are several facilities here to test your strength as a Pokemon master. The battle palace is just below the battle arena. The battle arena is to the right.
Get four miscrits and Go battle arena Then u saw four miscrit in ur enemies Then U can see what miscrit they are using then switch to The weaknest.. Tip:When Going to Battle Arena,Put a Fast Miscrit At the first slot.I recommend Dark Flowerpillar.