The best method for grinding EXP (Excluding the elite 4 as you probably won't clear it) is just fighting Pokemon on victory road. You can lead with a fainted Pokemon with the illuminate ability to increase the amount of encounters you get, while still leading with lairon. You will suffer EXP losses however, as you'll need to run or switch out against any hairyamas. This method with only 1 hit KO's should give about 2000 EXP per minute
If your lairon is too weak to easily defeat the Pokemon, you can passively level it up by throwing it in the daycare. It may lose important tm moves if lairon would learn a move, so do be weary of that. It only gives about 850 exp per minute, though this can be sped up by using speedup on an emulator.
Finally, if you're on an emulator, you can always just hack in 999 rare candies.
source: source