you cant you have to trade it and if you want to catch the other regis you have to regigigus in your party and then for regice go to end of mt coronet for registeel you go to cave of origin and take your mothers exit find the rest of the way yourself and for regirock you go to route 229 thats all i know
you cant get mudkip without trading on platinum
There is no way to get Bulbasaur in Platinum without trading or Pal Parking.
Cheating
you'd need a rebattle reggigias cheat from action replay if you have one.
no you have to trade him to evolve him
Without trading? well, then there's one solution for you:Hack.
That's impossible.
You don't. Its not possible
The only way to get Shaymin in Platinum without cheating or an event is through trading it with a friend.
With ans AR
Try the GTS trick.
You need to transfer the 3 Regis from Ruby, Sapphire and Emerald and take them to the Snowpoint Temple, Regigigas will wake.