You can either migrate it from Pokemon Emerald version, get it from trade,
cheats, or events. :)
You can't
On Birth Island.
darkrai will win.because darkrai have abilities to make a Pokemon having a nightmare. dexoys kk
you go to bith island or trade it on wifi
You can't without action replay
find the code online
You can find Deoxys in Saffron City where Sabrina is.
There is no best Pokemon on Pokemon HeartGold.
After you beat the game, there is a secret mission you can choose from the main menu
No only in wish island
there is no Pokemon Mansion in Heartgold.
They are not in Pokemon HeartGold