It's easy to do once you get the highest level of Lightning. There are a few places that have infinite enemies as well, so take full advantage of those and rack up points.
Are you aware of the experience glitch/cheat/ZOMG awesomenss? If you don't, than read on.
Eating a fish, meat, or carrot give you plus 3 for either Will, Strength, or Agility respectively. Combat multipliers effect experience gained. If you have a multiplier of 200+... you get the picture. Potions usually grant +1000 in their respective skill, but when multiplied, they can offer you thousands upon thousands of experience. To take even greater advantage of this, simply place all your potions/food on the quick slots and jam those keys down and watch as you become godly in an instant.