first you need to obtain the silph scope. you get that in saffron city, in the silpher co. building (its the biggest one). then when you go into the Pokemon tower, all the ghosts turn into either a gastly or a haunter. you can then battle and catch them with Pokemon.
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you need to find the silph scope this will reveal the ghost to be a marowak you can then defeat it
Get a Slith Scope and climb the towe. Beware: the ghost Pokemon that you visually see transform cannot be caught with a Pokeball
Go to the casino in cetrulon city (i cant spell) go in the casino go to the team rocket member talk to him he will move press the poster and the corner will open up go to the top and beat giovanni once you beat him get the siph scope and go back to the ghost you can pass now
You need the Silph Scope to see it, then defeat it.
Go to the Silph Co. in celadon and get the sliph scope.