If your in the battle frontier and have access to the battleground (bucks Grandfathers house), Equip exp. share or lucky egg (or both!) and fight Pokemon level 70+, that should get you to level 100 in a matter of weeks, if not days.
You train your Pokemon to lvl 100 or you can find some at the battle frontier in the place where you rent Pokemon and select open level in Pokemon platinum
train... train...TRAIN...
Level 100
i train my Pokemon to lv 100 without AR, i train my Pokemon on routes 212, 225 using the vs seeker hope i helped
e1337 4
100
Train your pokemon.
Arceus
I would use rare candies. Or, train in Victory Road.
Level 100 legitly.
It's a level 100 Magikarp.
You can't find level 100 Pokemon you have to train a Pokemon to that level yourself.