Go to the middle of the tile maze and open the door. Inside you will find Regigigas - the hardest boss of all. Why is it hard? Whenever he stops he creates a shockwave that takes off 18 hit points in one hit. He can also create four fissure pockets on the ground, make a fissure in the shape of an "x" which also drops rocks down and creates a massive hyper beam that damages you massively. The best strategy here is to bring electric and steel type Pokémon. The steel assist will make him unable to attack (that includes the shock waves he produces when he stops.) After that, it's easy to capture him as he's very slow.