Instructions
Catch a Wailord and Relicanth. Find Wailord near route 129 while surfing and find Relicanth near route 124.
Teach your Pokémon to fly, dig, surf, dive and use strength. All of these moves are represented by HMs or hidden machines or TMs or technical machines. Only one of your Pokémon has to learn one of the designated HMs or TMs. For example, if you teach your Wailord surf, then you do not have to teach surf to any other Pokémon. Find "Fly" from the Weather Institute, find "Dig" on route 144, find "Surf" in Petalburg City, find "Dive" on Mossdeep Space Station and find "Strength" in Vedanturf Town.
Visit Pacifidlog and head southwest. Dive in the shallow waters by accessing your menu and Pokémon then using the "Dive" option. While under water, head down and to the right to reach an area with some ancient writing on the wall. Press the "A" button to read it and access the surface.
Use "Dig" at the wall ahead to access another area. Read the writing on the wall to trigger an earthquake.
Fly to Dewford and head north. Go west of the island with plenty of trees, then head north again. When you hit land, enter the cave.
Walk up to the wall and read the ancient writing. Wait for three minutes for the door to open and walk in. Regice will appear right in front of you.
Press "A" to enter a battle with Regice. Attack Regice with super-effective fire attacks, such as flamethrower and fire spin until its hit points bar turns red. Throw ultra balls at Regice until you have caught him; this might take a couple of tries to complete.