Thanks guys!
I'm currently leveling them up on Victory road, but it's really long. Do you guys have any other places where I could grind faster?
Actually, on Route 222 over by the Valor Lakefront on the Beach, there's a Fisherman Trainer. At your point, he will have one Magikarp and one Gyarados. You're focus is the Gyarados; it offers a substantial amount of Experience, and it's not difficult to take out. With my Palkia in Pearl, I was one-shotting him with Spatial Rend. I would assume Roar of Time is Dialga's comparable move. I also got rid of my Palkia's AncientPower move as soon as I got something else. I believe I replaced it with Slash, it's a Normal move with 20/20 PP.
Now, the trick with this Fisherman.. Dialga and a strong Electric Type will likely be able to One-Shot this Gyarados.. Give whoever is in the lead a Lucky Egg, and get them to roughly 55, then switch out to your next one. If you're having difficulty with the Gyarados, opt for keeping a stronger Pokemon in (Dialga or your Electric), and give the one you wish to work on the Exp. Share. This battle is extremely easy, and yields a considerable amount of experience for minimal effort. And, you can run right over to the Lakefront Hotel and restore your Pokemon.
This tactic employs the Vs. Seeker, so running over to restore PP/Heal Pokemon works out well. Hell, with the layout of 222 and the Hotel being there, it's almost like this was expected to be used for this purpose. Now, the Fisherman you're looking for is Fisherman Alec. He's located closer to Valor Lakefront. Easy way to get to him; Fly to Pastoria City, run back to Valor Lakefront, and head over to Route 222, like you're heading to Sunnyshore City. As soon as you get on Route 222, you'll see a large patch of grass above you, with a Rich Boy Trainer. Descend the nearby stairs, and the /first/ Fisherman you see if your target. Fight him, and just keep using Vs. Seeker on him.
Once you get to the point where the Experience Yield is turning to crap, then venture back to Victory Road. Your team will be substantially stronger, and it won't seem as long or tedious to train them there. I employed the above mentioned tactic, and got my group to an average level of 57, and have no problems wiping the Elite Four and the Champion. In an hours time, with this tactic, I generated over 140k experience among my battle team; that's a pretty nice turn out. After you beat the Elite Four and the Champion, this same tactic can be used if you wish to take your Pokemon to Level 100, because the Fisherman now has two level 53 Gyarados, that with a Lucky Egg and Boosted Experience from Trading? Yields over 5500 XP per Gyarados.
I hope this tactic helps you out! If you have any questions, drop me a line. :)