You can't unless you get an action replay and enter cheat codes. Sorry dude.
Nintendo events
No
You can't catch it but only by Nintendo events or cheats.
gAME sHARK
Only through Nintendo events, of which there are few at the moment.
celebi is onlyobtainable throw Nintendo events
it's a place to catch darkrai, shaymin and arceus
You go and get a action replay and get the rest or get darkri with the event
You need a special Pokemon event.
Official events are the only way to catch Arceus.
you can't get mewtwo only by trading or Nintendo events
Not in the real world but it is available to catch in a couple of the pokemon games.