Instructions
Talk to Captain Stern in Slateport City after completing the storyline events at Mt. Pyre. Captain Stern is located at the docks, surrounded by members of the press. After talking to Captain Stern, go inside the building to find overhear Team Magma talk about their plan. This event triggers the opening of the Magma Base.
Travel to the Magma Base north of Lilycove City. The base is in a cave surrounded by water, so use a Pokemon with the Surf ability to travel through the water.
Enter the base and use the top teleporter device in the first room. Exit into the next room and take the next teleporter.
Battle the Team Magma grunts in the next room. Team Magma uses mostly Fire Type Pokemon, so prepare ahead of time by putting Water Type Pokemon on your team. After battling both Team Magma grunts, take the teleporter on the southeast side of the room.
Battle the Team Magma grunt in the next room and continue through the door to enter the final room of the base.
Defeat the first Team Magma grunt and then take the teleporter in the south part of the room next to the water. This will teleport you to the other side, where you will have to defeat another Team Magma grunt.
Follow the path to confront Magma Admin Tabitha. Defeat her Numel, Mightyena and Camerupt to complete the base. Exit the base by taking the teleporter at the end of the path. The Wailmer will no longer be blocking the path to Mossdeep City.