Here is my strategy for quick leveling (BW1/2)
1.Hatch the Pokemon
2. Teach it Substitute/Toxic/Protect (Drop Protect if you have more egg moves, don't use if you have a lot of egg moves you want to keep)
3.Go to Giant Chasm(Bw1)/Route 1(Bw2) and find a Audino. set up a sub t1, toxic t2, use combos of protect/sub until it dies.
4. Get tons of EXP (lucky egg reccomended.)