You can't. That is the simplelist answer. Without the mystery gift you can't access the Darkrai Event and since the Darkrai Event has been and gone, without action replay you cant get the mystery gift or cheat an encounter with a darkrai.
So to put it simply you have to trade with someone.
=Its not possible but getting aceus is without action replay=
You would have to get an action replay.
Darkria is a mystery gift event which has finished now so you cannot get it without a cheat system ,like action replay, that's how i got mine! ;)
Action Replay codes.
you use an action replay
you can if you have action replay or any game device but if you don't you have to get the national pokedex to get darkria
it's impossible without mystery gift action replay or gameshark
TO MY CALCULATIONS U TALK TO SOMEONE
you have to have the event or you can cheat with an action replay
You have to get action replay chip for ds
You would have to go to an Event where they had Mystery gifts. But the events are past, so you cannot get one without an Action Replay now.
you need an action replay