get it in mystery gift
Shaymin can only be found by using the Action Replay or participating events. In Pokemon Diamond, Pearl, and Platinum, the event for Shaymin had expired. Trading or using the Action Replay are the only ways to encounter it.
Either by trading, a mystery gift, "Oak's letter", or using the walk through walls cheat to get to the end of the last route, where you find shaymin.
Trading, Migrating, Catching the legit legendary Pokemon in pearl, etc.
nope
You can't without using cheats.
Transfer one from a Pokemon Black, White, Black 2, or White 2 game using the Pokemon Bank and Poke Transporter applications.
YOu CAN GET A SHAYMIN BY getting the preffesors oaks letter from an Pokemon event or by using an action replay. for more info go to yourpokemongo.webs.com
no
you can only transfer certain Pokemon by using the relocator
By fighting a lot of trainers, and wild pokemon.
You can catch giratina without any use of action replay in pearl.
there is a website called zimbio.com. all you do is enter the code and the id number given, go to the mining muesum with a open space and boom, i got like seven shaymin :)