You're sure weren't kidding when you said she knows nothing about raising Pokemon...
If you don't want to give up now, then really all you can do is just attack all of Wallace's Pokemon with your strongest moves. You will mostly have to rely on Girafarig and Swellow, since Sceptile's moveset is horrible and the other Pokemon are severely underlevelled. Also abuse your items - you will need them if you want any chance of winning.
If you keep trying but just can't beat him, then just lose on purpose so you can leave and work on your team. First, definitely give Sceptile Leaf Blade, it's a really good STAB attack. Next, you should level up the weaker Pokemon to level 50+ like the first three Pokemon. If you want to use Tentacool, evolve it and teach it Ice Beam over one of the attacks not called Surf. Hariyama can learn Brick Break and Earthquake, which are pretty good moves if you have the TMs. Beautifly isn't the best Pokemon, so you could replace it with an Electric type like Manectric or Raichu - STAB Thunderbolt will be VERY useful against the Champion!
So really, what you need to do is level up the weaker Pokemon and give them more variety in their movesets.