get a Pokemon that is strong against water. like: Electrick and grass. then train it so its about level 20 and knows the types tht work well with it. then get Pokemon to beat her/his other Pokemon she has.
fight and ride!
you must go down cycling road and head for hearthome city!
It depends on the levels of your pokemon, and the ones Wally has. This is in Victory Road, so I assume he has level 40-50 pokemon. If yours are around this nature also, you shouldn't have a problem. But, seeing as you said Marshtomp, instead on Swampert (the final evelution), I'm worried your pokemon are much to low to even be at Victory Road. Anyway, the NPC's of pokemon are always using terrible moves etc, so it should be a breeze.
first go to the casino beat out team rocket and you'll get the silph scope go to lavender town and beat them again at ghost tower then you'll get he poke flute and you can wake up Snorlax and then you can access cycling road to Fuchsia city.
you have to talk to all the people in Eterna city after you beat the gym leader.I think he is particularly on the middle road that leads downward to cycling road
If your starter was torchic,she'll have a Shroomish,Numel and a marshtomp. I'm not sure about the other two because I have emerald. If treecko,then Slugma,I think shroomish and combusken. If mudkip,I think wingull,slugma and grovyle.
You get it at wayyard cave below the cycling road.
The 'Cycling Road' is between Eterna and Oreburgh (it doesn't go directly to Oreburgh, but that is the nearest city).
its all in the timing
wake the snorlax at the top of cycling road with the pokeflute then down cycling road to fuschia
Cycling Road can be found on Route 17 (East of Fuschia and Celadon).
under the cycling road inside the wayward cave or at the fight area when you beat the elite four and get point card and get 48 battle points.