Verified answer

The best thing to do when you feel your Pokemon levels are low is to train on wild Audinos. They give a lot of xp every time you defeat one and the are also relatively to beat. To do this, go to the nearest patch of grass, (Routes 5 or 3 both work,) and run NEXT to the grass until the grass starts to shake. 85-90% of the time it will be an Audino.

Pokemon Gold/Silver and Crystal are better. The new generation of pokemon makes me want to die.