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Well, I did it on FireRed, but was using an emulator, so...
I caught almost all of them, and got a bunch of others from saves that I found. After that, I breeded all of them, so they would have my OT and ID. All the legendary ones I had to catch myself, so they would at least have my OT. All Pokémon in my collection were catched using a normal pokeball, including the roaming dogs.
Mew is legit and I got it from the Distribution ROM that was leaked on internet early this year. Celebi and Jirachi are legit too, but I wasn't the one who "got" them. Celebi is a dump from a 10 ANNIV event, so I'm not the only one with this specific Pokémon, and my Jirachi is from the Colosseum Bonus Disc, but since I have no way to emulate a Gamecube here, I got one of the Jirachis that were already generated and dumped.
Lugia, Ho-Oh and Deoxys are all legit too. I injected the Mystery and Aurora Ticket from original dumps that were reverse-engineered a few years ago.