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.
it's a place to catch darkrai, shaymin and arceus
celebi is onlyobtainable throw Nintendo events
gAME sHARK
Only through Nintendo events, of which there are few at the moment.
You need a special Pokemon event.
You go and get a action replay and get the rest or get darkri with the event
you can't get mewtwo only by trading or Nintendo events
Official events are the only way to catch Arceus.
Not in the real world but it is available to catch in a couple of the pokemon games.