lick,curse,hypnosis,dream eater etc. Dream eater is the signature of gastly but it work only when the opponet is asleep. That means you have to hypnosis the enemy first then use dream eater.
He is in canalave city. To get there you need the HM surf. You can get it in celestic town. If you want to delete some moves, be ready to get rid of HM moves.
beat the pastoria gym leader then follow the galactic grunt near the great marsh then battle him at the valor lakefront then when u beat him cyintheia comes out of nowhere and gives u the a secret...