He will occasionally appear in front of the Battle Tower entrance. You know, right next to the Pokemon center. Be sure to battle him if you see him, because he'll only stick around for 2 days and then go away for awhile (say, 3 days).
He will actually only appear on Saturdays and Sundays.
But if you save the game while standing next to him, he will still be there on the weekdays unless you go some where else then come back; he will be gone.
he might not be there though so don't always think he will be there on weekends "ALL" the time
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Go to the fight area duting the weekend or Mount Stark in the week and he will be there to battle with you. he'll have 6 Pokemon so be ready.
He is waiting in the battle area to the right of the Pokemon center.
He will just pop up anytime in the battle zone, in front of the gate to Battle Park.
every weekend he will be at the fight area for a battle
your rival has one. if you battled him everytime then battle him in the fight area. if you want to use that catching people Pokemon cheatr then you can also catch a grotle from some person in route 210.