Are you planning to evolve Zweilous before the E4? Or on the way to N?
Anyway, I trained my Excadrill in Victory road to level 62, and got kinda bored. XD So it might be easier to train Zweilous to level 62 or 61 before challenging the E4. The main flaw with my method is Serperior has trouble catching up. So, especially with that move set, it will only get to maybe 52ish. Though that's around the level most of your Pokemon should be.
So, on to the method. Go back the way you came from in Victory Road, till you get in the room with the doctor. That's where I train. The Durant are the main EXP givers, but at most, they give around 1400. Next would be Boldore. They're annoying with Sturdy, but it's another option since you won't find Durant every time. Plus, that's pretty much the only way to train Serperior, unless you want to Audino train it somewhere before Victory Road. Since you have a doctor near you, and it's easy to find Durant and Boldore, easy training. Though it sometimes takes patience. You will also find Excadrills from time to time, burrowing in the caves, these also help training. Mienshao and Woobat also appear, but just run from them, don't waste time on them. It looks like all your Pokemon can easily take down a Durant (besides Serperior), so this should be a breeze for you.