If you're meaning Magnemite or Magneton, than you would be best using a rock type since they are immune to electrical attacks. Additionally, you could use a pokemon that is relative to fire, since both magnemite and magneton are electric-steel type pokemon.
Hope this helps!
You can beat a ghost type with another ghost type (Like Haunter)
I would choose a Dark or Ghost type
any dark type move
You have to beat all of the Kanto Elite Four. After you beat them, Lance will give you a magnet that allows Magneton to evolve.
Rock type Pokemon are strong against flying, ice, bug and fire type Pokemon.
You can either find them on some certain Electric-Type Pokemon,like,Plusle,Minun,Magnemite,and maybe Voltorb.I think there is only a 5% chance of it holding a Magnet. Or beat the Trick Master's 3rd Puzzle in the Trick House.the prize you will get is a Magnet. When held by a Pokemon,the Magnet raises the power of its Electric-Type Moves.
Ground type Pokemon can beat Electric type Pokemon easily.
You can beat it with Ground type Pokemon or Fire type Pokemon easily.
You can beat a ghost type with another ghost type (Like Haunter)
thunder pokemon
No type can beat the other types in pokemon they each have strengths and weaknesses and not one can conquer the rest.
Dark type can beat ghost.
beat the Trick Master's in the Trick House. he will give you a Magnet, i don't remember which puzzle.Read more: On_Pokemon_Emerald_how_do_you_get_a_Magnet
Ground
ghost
After you beat the Pokemon League you can travel to the Kanto via the magnet train in Goldenrod City
Rock, electric and ice.