How do I get Regigigas in Pokémon ORAS? [duplicate]

Taken from here.

Requirements

Have Registeel, Regice and Regirock in Party. Have Regice hold a specific Ice item; plus give it a nickname. The last Regi legendary Pokemon – Regigigas – is precarious Pocket Monster to catch. I can’t recall any Pokemon with so many complicated and specific requirements in order to be caught. With this said, it would appropriate to write up a quick post detailing the exact steps and requirements in order to catch Regigigas in Pokemon Omega Ruby and Alpha Sapphire.

Step 1: Obtaining the Regi Triplets: Registeel, Regice and Regirock

Your first round of orders is to catch Registeel, Regirock and Regice. There are a number of requirements needed to catch the three Regi legendaries. For the complete and most comprehensive guide on how to catch Registeel, Regice and Regirock check out Nintendo News Fix’s guide. Also you have to defeat/catch Primal Groudon/Kyogre the first time you encounter them in the Cave of Origin, located in Sootopolis City.

Step 2: Naming your Regice

The second step is that you must give your Regice a nickname. If you caught Regice and did not give it a nickname, you can give it a nick by visiting the Name Rater at Slateport City.

Step 3: Go to Pacifidlog Town

Fly to Pacifidlog Town and go inside the house right of the Pokemon Center. Talk to the girl inside and she will hint that your Regice must hold either Casteliacone, Nevermelt Ice or the Snowball. Pay attention to the conversation and decipher what item is needed.

• Casteliacone can be found in the Trick Room at Route 110 or be bought from the little girl at the second floor of Mauville City.
• Nevermelt Ice can be found at Shoal Cave.
• Snowball can be purchased at the Battle Resort for 32BP.

Once you have the item, have your Regice hold the item.

Step 4: (Optional) Wait

If you want to catch Regigigas, you also have to wait for it to be day. You can't encounter this legendary unless it is day-time. If it is already day-time, move on to step Five.

Step 5: Catching Regigigas

Now that you have the three Regi’s with all the prerequisites completed. Head back to the Island Cave where you caught Regice, which is Surf North of Dewford Town. Enter the cave and head to the second room. Now, if you have done everything correctly; an earthquke will strike with Regigigas challenging you. All the basic principles of catching legendary Pokemon apply here. Reduce the HP down to the red and apply a status effect, (like paralysis or sleep) then proceed to throw Ultra balls.

According to Bulbapedia:

In Pokémon Omega Ruby and Alpha Sapphire, the player is able to encounter Regigigas in the Island Cave after capturing Regice. To do so, the player must have a nicknamed Regice holding a "cold item" (a Casteliacone, a Never-Melt Ice, an Icy Rock, or a Snowball), as well as Regirock and Registeel in their party. Regigigas cannot be encountered during nighttime. As Regigigas is not in the Hoenn Pokédex, it can only be battled after capturing/defeating Groudon/Kyogre.