Note: you need acro bike
Put two of your pokemon in the daycare and plug your charger into your gba. Go to fallarbor town and go all the way east until you see a sand hill thing (route 111), use your acro bike stand right in front of it and keep trying to go up it overnight, good luck finding something to hold the up arrow =]
Happy lvl 100 pokemon
Note: Might take at least 1 1/2 days.
My pokemon gain at least 70 lvls a day from overnight and school.