From what I've seen the best place to level is Route 20. You'll need a pokemon with an Area of effect attack similar to pedal storm. (there are ghost-grass. and grass poison pokemons there so keep that in mind.) Next you'll want to obtain a combee that knows sweet scent and has the ability honey gather(so you can eventually leave it in the pc) Next get the lucky egg, go to coumarine city, take the train, then go to the hotel, once there talk to a lady with a pokemon that has high friendship (your starter should work.) Next obtain the exp O power, forgot where I got it from, but its 25% bonus exp for level 1, level 2 is 50%, i havent gotten level 3 yet.
So once youve obtained all this, activate your O power, remember it only last 3 minutes, put the lucky egg on the pokemon with the AoE. Then Use sweet scent(combee-move) and then kill them all at the same time for bonus exp. Also sometimes rare pokemons mix up with the group and shinies do spawn in hordes so be on the look out!
Hope this helps you out
