Do you mean how do you pass the Fight Area?To do this, you have to get the National Dex by seeing all 210 Sinnoh Pokemon.You don't have to have them all, you need to have at least seen them all.This...
First you have to have 16 badges
you need to beat 21 battles but i think you do a 100 streak i know he has milotic and rhyperior but the other one i forgot.
you need all Pokemon in you pokedex ...
it depends on what game like in leaf green and fire red you can get it one of the 7 islands