Instructions
Trade for the fateful encounter Regigigas in "Pokemon Platinum" if you didn't obtain one from the Toys"R"Us event. Toys"R"Us held the event from March 8, 2009 to March 21, 2009. Since the fateful encounter Regigigas is an Event Pokemon, you will probably have to give up rare Pokemon for it, or settle for a hacked one.
Place the fateful encounter Regigigas in your party. Get a Fire- or Fighting-type Pokemon and train it to level 25. You will use this Pokemon to weaken the Regice, so you will have a greater chance of capturing it.
Sell valuable items like nuggets and fight Pokemon Trainers for PokeDollars. Purchase Dusk balls from a PokeMart in Sinnoh. Since Regice is in a cave, a dark environment, these Dusk balls will prove more effective than Ultra balls.
Climb Mount Coronet and look for a cave at the Iceberg Ruins. Enter the cave and step on the seven dots on the floor that are in the form of a plus sign. Your character will notice a change in the atmosphere. Walk up to the statue to engage the level-30 Regice in a battle.
Fight the Regice with Fire- and Fighting-type attacks. Once it is weakened, attempt to catch the Regice with your arsenal of Dusk balls. If you have a free space, the Regice will be in your party. Otherwise, the Regice will go straight to your Pokemon Storage System, where you can retrieve it.