Both other solutions here give a good way to grind out this achievement on its own. In my experience playing through the game, I noticed there's a shit load of grinding to get to levels, so I tried to simplify and make more efficient the methods in place.
When you are going for the max all character levels at 99 achievement, and you are most undoubtedly doing the "Golden Poo Snake" method of gaining over 1k xp per quick battle, you might as well knock this achievement off while you are at it.
The Golden Poo Snakes lunge quite a fair distandce towards you if you leave yourself idle before them for a split second or two. This leaves them extremely vulnerable to running around behind them and initiating an attack from behind. They even have a more simplistic attack where they just swing their weapon at you, leaving them vulnerable as well, but for a smaller window. I found that the initial boredom of levelling to 99 evaporated as I was working towards two (seven if you count shadow levels to 99) achievements at the same time. (Initial, because you'll have to initiate more than 100 battles if you want to max out your character levels.)
So this is my suggestion, kill two birds with one stone, and don't find yourself running around with stink on after you've beaten the game, trying to catch pathetic little creatures as they run away.