Well first, You're pretty underlevelled. The champion's strongest pokémon is a Level 58 Milotic so you'll definitely want all of your pokémon to be in the 50s, probably close to 60s due to your weakness to Ice.
I would suggest ditching Marill and picking up a Lairon in victory road. You can get a low 40s level Lairon fairly easily in Victory Road. Then evolve it into an Aggron.
I'd recommend:
Aggron
Brave Nature
Level 50+
- Thunderbolt (TM24)
- Ice Beam (TM13)
- Brick Break (TM31)
- Earthquake (TM26) or Iron Tail (TM23)
Brick Break will easily handle Dark and Ice types. Not to mention Aggron is resistant to Ice. Thunderbolt will help with Water and Flying types, but Aggron is weak to water types so watch out for that (A lot of the ice types can learn water type moves as well so be careful). And then Earthquake gives you a solid, powerful phsyical attack to go with Aggrons high attack stat, or if you already used the Earthquake TM (I think you did on Cradily), you can go with Iron Tail. It's strong, gets a STAB bonus, but it's also inaccurate. .
At level 47, Dodrio learns Drill Peck (80 Power), which is stronger than Fly (70 power), so go to the Move Deleter in Lilycove and delete Fly and replace it with Drill Peck. Definitely keep pursuit because that will help against Pheeobe's Ghost types. I don't really know what else you can do with him.
New moveset:
- Pursuit
- Steel Wing
- Tri Attack
- Drill Peck
Sceptile needs some serious work with his moveset. Leaf Blade is good, but everything else has got to go. Teach him Dragon Claw through TM02, because this will help against Drake's Dragon Type team (along with Aggron's Ice Beam). I would replace Cut with Solar Beam. Assuming Sceptile can take a hit, it'll be good for OHKOing Wallace's water types. Then give Sceptile Toxic as well, as this will badly poison enemy pokémon, giving a bit of an edge. You could also go with Strenght, since it's a decent attack.
So Sceptile's new moveset could be:
- Leaf Blade
- Dragon Claw (TM02)
- Giga Drain (TM19)
- Toxic (TM06) or Strength
I think Cradily and Salamence should be alright. They've got solid movesets. Just level them up and they should be good to go. There's also not a whole lot you can do with Walrein either. Sorry if I suggested TMs you've already used. Idk what items you have and which one's you don't :P Hopefully my suggestions will help a little. Good luck! :D