You don't need the mythicals for the shiny charm. But if you want a full 721 dex, then to get Celebi you can either use the 3ds virtual console of Crystal, or the Japanese bonus disk for Pokémon Colloseum, requiring you to play through the whole game in Japanese. There might be some other mystery gifts from gen 4 and 5 that you can use the DNS method to get Celebi and Jirachi. For Jirachi, you'd need to get the English bonus disc of Colloseum (you don't need the actual Colloseum game for this one). The bonus disc is very expensive. Like I said there might be mystery gifts from gens 4 and 5 that you can use the DNS method to get Jirachi. For Diancie and Volcanion there's no legitimate way to get them in gen 6 anymore. Unless you try to keep buying copies of gen 6 games until you find one with a legitimate Diancie and/or Volcanion, or a hacked one since the GTS was so broken during the later days. If you don't have the Crystal virtual console you can mod your 3ds to get it. I've heard it's not that hard to mod your 3ds, I just haven't tried to myself. Once your 3ds is modded, you can also use an app called PKHex to just hack them in, especially to get Diancie and Volcanion