Just when you think its all over, captured all 266 Pokémon and are sure that it is most definitely over, you get reports that a new section of Hippowdon Temple is open and it needs the three legendary golems; Regirock, Regice & Registeel to open up further. Go to the middle of the tile maze and down with them and open the door. Inside you will find Regigigas...the hardest boss of all. Why is it hard? Whenever he steps he creates a shockwave that takes off 18 hit points in one hit. He also can create four fissure pockets on the ground, make a fissure in the shape of an X which also drops rocks down and also creates a massive hyper beam. The best strategy here is to bring electric Pokémon to paralyze and damage and Steel Pokémon. The Steel Pokémon assist will make him unable to attack and that includes the shockwaves he creates when walking. After that, its simple to capture him as he's so slow. Once done, be happy as you've completed the game.
Hope I helped :)