Train or use the VS seeker.
you train them
train in wild Pokemon battles through out the game
what does that mean it means your mother. How do you get staraptor on Pokemon diamond without cheats is the proper way. Catch a Starly, level it up to evolve into Staravia, then level it up until it evolves into staraptor.
but only without AR no cheats!
you must be a very patient person :) THERE IS NO EASY WAY :(
no not really unless you mod the dsi itself along with cheats
it's a bit hard to get pokemon up to level 100 if you can't cheat nor leveling up. wifi events pokemon are usually lvl. 100
seek for wondermails with joy seeds
Get cheats and get any Pokemon any level
You cannot. Whatever you can do in the game naturally (i.e. leveling up normally, by battling and using the rare candies available) is all that you can do to level up. The only methods to level up faster involve cheats.
the fastest way to level them up is to take them to an area that has Pokemon leveled at or slightly below the level that your Pokemon is also if you battke with exp. share on your Pokemon it gains double exp.
to "obtain" 100 lvl pokemon without action replay or any cheats, you have to do it the old fashioned way and train it by battleing other pokemon with it, otherwise there is no other way unless you trade