Yes First, you need to have a Regirock/Registeel/Regice in your party.
you cant. regice regirock and registeel are all part of the hoen pokedex
yes
get regice regirock and registeel and go to snowpoint temple
Kyogre and regirock-regice-registeel
Sorry but you have to catch Regice, Registeel and Regirock in order to catch Regigigas.
Regice, Regirock, and Registeel are unobtainable through normal gameplay in Pokemon X or Y. The only way to get them (without cheating) is to receive them in a trade with another player or transfer them through PokeBank/PokeTransporter from an old game. As of September 19th, 2014, there have been no events announced where you can get Regice, Regirock, or Registeel.
You can't... without Action Replay. I do not know a legit way to get Regice (along with Registeel and Regirock) The only Regi I know how to get legit is Regigigas, which needs Regirock Regice and Registeel. If you know a legit way to get Regirock Regice and Registeel, edit this answer. actually you can. all you have to do is send Pokemon from ruby, or Sapphire to dimond or pearl or platinum. then trade it to soul silver.
You can only get regigigas only after beating the elite 4 and getting the national dex. It will be in snowpoint temple and you have to have registeel,regirock and regice in your party.
You have to get it in Pal Park from R/S/E.
Use Gameshark or Action Repley codes
Regirock, Regiice, Registeel, Rayquaza, Kyogre, Groudon, Latios or Latias
Giratina - Turnback Cave on the spring path Heatran Palkia/Dialga - Spear Tower Azelf/Uxie/Mesprit - At all 3 lakes inside the caves in the middle Regigias - If you can obtain Regice, Regirock, and Registeel and have them present in your party. Rotom - Inside the house in the Old Chateaau