If you have a bike, (which you have to get by having the bike voucher and going into the bike shop in Cerulian City) then go to cycling road next to Celadon City. Once when you get passed cycling road, you get into Fushia City. The gym leader in Fushia is Koga (poison type leader, I recommened grouned type pokemon).
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After you beat Sabrina, the 6th gym leader in Saffron City, you can use Rock Smash.
well she isn't a boss. shes the gym leader. you have to beat the twoer of team rocket then go into her gym and beat the pattern of teleporter pads.
The marsh badge is in the saffron city gym you must beat Sabrina the gym leader to get it, but to do that you must beat team rocket in the silph co building.
Well I have pearl. But when I went to one gym she was not there because I had to beat a different gym leader. So you might want to try beating a different Pokemon gym leader........ Hope I helped..... Babygirl14000
The sixth gym leader,Sabrina, is in Saffron City.