1.mirgrate the three regis from your Pokemon games put the GBA pokemon in the gba slot and the pokemon diamond/pearl in the top(note:make sure you have six pokemons in your battle team to mirgrate them)
2.now go to pal park after you mirgrate the pokemons
3.now talk to the guy in the counter and youll get a certian pokeball
4.now find the regis
5.after that go to the snow temple the lady blocking it will not let you enter
6.the gym leader there will come and let you enter
7.go to the very last floor now get Regigigas(note:bring the three Regis to awaken Regigigas)
8.now damage it till its to 78 or 87 poison it and put it to sleep then try to catch it with a ultra ball it might not work but just try
9.then you have Regigigas
(Note:To make this work you have to beat the elite 4 and obtian the National Dex first)