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 either migrate it from Pokemon Emerald version, get it from trade, cheats, or events. :)
You can't without action replay
find the code online
You can find Deoxys in Saffron City where Sabrina is.
No only in wish island
After you beat the game, there is a secret mission you can choose from the main menu
The only way to obtain a Deoxys is to migrate it from Pal Park.
You cannot catch Deoxys in the wild. It'll need to be migrated.