No, Shaymin can't be caught though normal game play either. You have to get it from a special event from Nintendo. You can see when these special events are by checking it out on the official Nintendo website.
complete the pokedex get all badges beat battle tower etc
You Have To Have Seen Or Catch All The Pokemon
trade or catch one
A big Wahoo and a Super Rod to catch Lumineon!
No, you have to get Shaymin by event.
im pretty sure after that you'll have to complete the national pokedex
You have to catch 151 pokemon to complete the pokedex.
I will list the Pokemon and how to get it. Giratina Reverse World Shaymin Cheat,Event Darkrai Cheat,Event Rotom Haunted Mansion Moltres Complete Pokedex Zapdos Complete Pokedex Articuno Complete Pokedex Manaphy Manaphy Egg-Cheat,Event There are others, But These are the ones I know.
Find all 210 regional Pokemon to complete the Sinnoh Pokedex. Then, talk to Professor Rowan in his laboratory.
complete the pokedex.
Complete the national pokedex (catch 200 pokemon) you can also get Cyndiaquil or Chikorita but you can only choose one :(
Basically, the only things you can do are catch rare Pokemon and attempt to complete your national pokedex.