just breed your charizard with a ditto and hope for one with a better nature. seriously, it won't take long. a fast way to level up is to always enter battle with the pokemon you want to level up(say enter a battle with a lv 5 charmander against a level 55 onix), and then switch to a stronger pokemon(your high level charizard). when the battle is over your charmander will still get half the experience and should level up hella fast this way since high level pokemon have a lot of experience points.
you have to start over anyway, if you want to give your charmander the proper EV's. but i don't get why you'd hate training so much because once you beat the game you can battle any level of pokemon that you want(just go to the proper area) and the pokemon that comes from breeding are usually easier to train because they can have stronger nature as well as egg moves...