After you defeat the champion at the pokémon league go to Mossdeep city to Stevens house and there is a pokéball on the table that contains a Beldum a Steel/Psychic pokemon that already knows the move Take Down give it exp.share that you can get in Rustboro city from Devon.corps after you deliverd the packedge to Capt.Stern just talk to M.Stone the president of Devon Corps train Beldum with the exp.share to level 20 and it will evolve to Metang also a Steel/Psychic pokemon and it will learn the moves: confusion and Metal claw train it further to level 45 and Metang will evolve to Metagross a very strong pokemon of the types Steel/Psychic.