Keep using Marshtomp's Mud Shot, it will lower their speed, and then you should use Water Gun.
yes
there are no specific Pokemon that can automatically beat the elite four. HOWEVER, ( i speak from experience) i beat the elite four with one Pokemon whitch was Samurott. i suggest using Samurott 2. hope i helped! :) - Raven
You can beat Skyla the gym leader with a higher level pokemon or with electric or rock type pokemon. And I do not reccomend using grass or bug types.
Hope and pray while using a ice/grass pokemon
Wally's dad lives in the house to the left of the Petalburg Gym, in Petalburg City. After you beat your father (Norman), he will give you the HM Surf.
It would be hard to beat him without evolving it in to swampert first, because then swampert can learn muddy water. But marshtomp's most effective move should be Mud shot.
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.
After you beat leader Norman
He is an electric type trainer so you will want to use rock or ground types and avoid using flying or water types. However if you have a Marshtomp you should be okay to use it as it is part ground.
You beat Leader Norman, then someone gives you surf.
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.
Wattson is easy as long as you have a ground type Pokemon like Marshtomp and use ground type moves like mud shot or have a grass type Pokemon like Grovyle or Breloom.
I would say any fighting types.
my marshtomp beat the elite 4 on its own at level 60! and it had hyper beam surf waterfall and muddy water
all lv.100s because it smacks the hardest way is to all the same lv.s as Norman
Norman has a Slakoth (Level 28), Vigoroth (Level 30), Slaking (Level 31) so I suggest you train a party of mainly Fighting type Pokemon to an average of about level 30 before you challenge Norman. Good luck!
at least level50.otherwise you will not be able to defeat his slaking.