Regigigas is the Frankenstein (or shall we say, frankémon) of all the Regi golems, it takes Regice, Regirock and Registeel (the Regi's), binds them together and creates…itself, Regigigas.
The only way that you can catch Regigigas is if you caught the Legendary Trio: Regice, Regirock and Registeel (here is the how-to on grabbing the three).
Once you have those guys, take your party down to Pacifidlog Town. In the North-East part of town you will find a house with a young girl in it. The little girl will tell you the legend of Regigigas (and her grandfather's stories – so – if you don't hear about it right away don't worry), the story will tell you a rough 'how-to' on capturing the frankémon.
After having a word with the girl in Pacifidlog Town, grab your Regi Pokémon and head to the cave (using Soar) where you grabbed Regice on Route 105 (North of Dewford Town). At this point, make sure that you nicknamed Regice while it was holding one of these items: CateliaCone (obtained at the Trick House), Snowball or Never-Melt Ice (in Shoal Cave). If you didn't nickname it, there's still a chance for you to grab Regigigas — just head to Slateport City and go to the Name Rater (you can nickname Regice there). Head into the cave during the daytime, survive some earthquakes and then be challenged by Regigigas.
Here's where you can find more guides for catching legendary Pokémon and finding Mega Stones.
