no its inpossible unes you've got cheats
you have to catch all of the pokemon in diamond or SEE all of the pokemon
you cant you can only cacth regice regirock and registeel.you cacth regigigas by migrating the regis to diamond pearl and platinum.In platinum hes at lvl 1, in pearl or diamond at lvl70. hes at snowpoint temple that the girls blocking. Bring all three regis and she lets you in.
you have to see all the Pokemon in sinnoh.
you have to see all the Pokemon in the pokedeks
What you have to do is catch all the Pokemon from all the other regions(complete national dex.
get all the legendarys
well you need to have all your Pokemon in your pokedex and when you get your national pokedex you will get breed
with an AR
No is doesn't only if you complete the national dex
By seeing all of the Pokemon there are in Sinnoh.
You can get the National Pokedex in Diamond when you see all 150 Pokemon, but not necessarily capture them.
Catch all the Pokemon.