by using wifi or wireless at a certain place
I am afraid you can not.
special events
no. only events
A hack of Pokemon SoulSilver that makes the game harder and adds new Pokemon not in SoulSilver, along with new game events.
6.
Celebi is only available through special Pokemon Events.
It's impossible to get without certain events/Hacks. Sadly, all of the events are over.
Action replay or Nintendo events
They are special events at stores like Toys'R'Us and GameStop. Special Pokemon like Arceus, Darkrai, and Celebi are distributed through them.
Usually Arceus can only be obtained through Pokemon events, and all the soulsilver events passed :(
Transfer it from your bottom game or get it from a Pokemon event. Pokemon events are under Wifi Conection but they don't happen often.
None. All of the Pokemon HeartGold and Pokemon SoulSilver events have ended, no more events for those 2 games. ~ 16 December 2012