From Bulbapedia, the community-driven Pokémon encyclopedia.
Overview
For those of you who have in their possession an event Regigigas in your party it is possible to be able to collect Regirock, Regice, and Registeel in your game without having to import them from Ruby, Sapphire, and Emerald through Pal Park. They are notably the only non-event legendaries that cannot be found under normal conditions in the Generation IV games (similar to Ho-Oh and Lugia in the Generation III hand-held games).
The requirement is that you need to beat the Elite Four first and get the National Pokédex. Normally when you enter their chambers it is an ordinary room that has a Type-enhancing item. But with the event Regigigas these rooms become the shrines where these three Legendary golems are found. Unlike the other legendaries that re-spawn after defeating the Elite Four should they faint before successfully catching them, then exit the ruins, return, and start over again with their puzzle.
By catching the three golems, you can also catch a regular Regigigas at the Snowpoint Temple. While you may only be able to catch them once per game, if you trade the event Regigigas to another game before deleting your save file and starting over (CAREFULLY DOUBLE-CHECK TO AVOID ACCIDENTALLY DELETING IT) you can collect more copies of the three Legendary Golems for yourself and your friends.
Before going after the golems make sure to stock up on Dusk Balls. The Ultra and Timer Ball can also help. If necessary Fly to the Pokémon League to get more since all the special Poké Balls are sold there. Make sure to bring plenty of healing items as well and remember to SAVE before each battle. It would be a good idea to bring Scizor, Forretress, Metagross, Wormadam (Trash Cloak), or Bronzong along since their unique type combo and high defenses are helpful to withstand the Legendary golems attacks if you need time to catch them.
Around the northern area of this route which is accessible with your Bicycle around the area which is surrounded by dirt mounds is where you can find Regirock's cave. For help with preparing your team, it would be a good idea to do it at the Resort Area as it is easier to start from and not as many HM abilities are needed to get to the cave.
Rock Peak Ruins
Step on the dots representing the marking on it's face and then a minor tremor will occur. Then interact with the nearby statue to face Regirock.
Paralyze it at once and prepare to lower it's HP to the red zone. If your team is too high-leveled just use False Swipe to weaken it. Using an attack raising move like Swords Dance will also help. If not the only thing to watch for is it's ability to use Curse which can raise its Attack and Defense stats by one level at the cost of one level of speed. This can be trouble especially since it has the highest attack power out of all the Legendary golems as well as the highest defense. Also this move can strengthen its strongest move Superpower, though that move also has the flaw of undoing Curse's strengthening effects. It does have the weakest Special stats and with its Sp. Defense, minimize use of exploiting that unless if you have a Pokémon that won't be too much of a risk of accidentally knocking it out.
From either Eterna City or Celestic Town go through the north area of Mt. Coronet by using Strength to push the boulders around and use Max Repel to avoid any wild Pokémon. Once you descend to the BF1 use Defog if want and head for the other side and go up the stairs. In the room that has the exit that leads to Route 216 is the entryway to Regice's room.
Iceburg Ruins
Just like before touch the dots and then interact with the statue to face Regice.
Paralyze and False Swipe Regice right away as it does have the weakest defense. Otherwise weaken it as much as you can with Special moves or a physical move that is not too risky as long as it is not super effective. You won't have to worry about the Curse/Superpower Combo as much since it does have the weakest attack stat. While it does have a high Sp. Attack, the only Special move it has is Icy Wind which you can use a Fire, or Steel-type to guard against. You can also use Special moves to further minimize the risk since it does have the highest Sp. Defense.
From Canalave City catch the ferry to Iron Island. Using a Max Repel to avoid any wild encounters, travel to the end of the maze and in the last room you will find the entrance to Registeel's chamber.
Iron Ruins
Just like the other two ruins touch the dots, then interact with the statue to face Registeel.
In terms of base stats, Registeel is the balanced one out of the legendary golems, giving an average set of physical and special stats when compared to Regirock and Regice where it has none of their strengths or weaknesses. Just like with Regirock, using a Attack stat raise/False Swipe combo can help weaken Registeel. Othewise a good balanced team will do the trick.
