Buy the dragon armor and equip it. You can also do the same for the flame mail and flame sword. If you play the fire abyss you will see that you have two circles inside each other beneath the fire icon at your status screen. This means they can't hurt you. Instead you gain life if you get hit. This is a very easy way to get XP for none of the enemies can hurt you.
However, the boss is different. He can still hurt you by jumping onto you, and if you hit him with a fire weapon he'll also gain life. so buy some mp potions and use some earth spells on him.
This is just one nice and easy way to gain experience.