You'll need to trade from Pokemon Diamond, Pearl, or Platinum.
trade from platinum,diamond or pearl. GTS. it can't be caught within HG/SS yet
You go to Union Cave and it will appear
shaymin
in yhece3vbggd21ecgy6
You can't get the Miracle Berry without the Shamin event.
Yes but at end of the game
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no
If by "shamin" you mean "Shaymin", then you can't. It must be traded from Pokémon Diamond, Pearl, or Platinum.
heatran, palkia,all the lake pokemon, shamin
a special event
There is no best Pokemon on Pokemon HeartGold.