If you want your low level Pokemon to level up as fast as possible, you should give it an exp. share, use it in a battle against another trainer's high level Pokemon, and then immediately switch to your high level Pokemon. Make sure your battle mode is shift, not set, so you can do this multiple times in one battle. Trainers' Pokemon give 1.5 times as much experience as wild Pokemon if all the other variables (species, level, etc) are the same.
If you want to EV train your low level Pokemon in HP, you can run around in Rusturf Tunnel (100% Whismur). If you want speed, you can use an old rod or good rod on Route 104 (100% Magikarp) or run around in Altering Cave (100% Zubat). If you want any other EVs, I don't think there's a good EV training place with low level wild Pokemon.