First, get an Eevee from Bill's house in Goldenrod city. Next, capture a Ditto in the grass South of Goldenrod city. After that, evolve the Eevee into an Espion (i.e, by making it happy in the morning or day and then making it grow a level) and teach it TM Shadow Ball (get it by defeating Gym Leader Morty). Now train the Ditto up to around about the same level as the Espion and put them both into Pokemon Day-Care. Wait for the old man to give you an egg (check the day-care centre as much as you can) and wait for it to hatch. The egg will hatch into an Eevee that should know Shadow Ball (A Ghost-Type attack). Now you can use a ghost attack against most enimes and most enimes won't be able to use ghost attacks against you!