At some point you should be able to get your hands on two pieces of equipment called Gator Bite and Demon's Tail. They have very useful abilities if you know how to use them.
Gator's Bite has the special ability called 'Devour Enemy'. What this does is the first important part of this trick. It doesn't work like it says it does. 'Eat dying enemies to gain their skills' is kind of misleading. You have to use the special attack it grants, which is called 'Munch'.
Demon's Tail has the second important part, the ability 'Subconscious'. This grants one point of EXP every turn, whether you move, attack, charge a special attack, whatever. Each turn gives one point.
The third part is the single most important part of this trick. Find a cactus type enemy, preferably in the easiest dungeon to capitalize the best on this, and there should be no other enemies if it works right. They have the ability 'Photosynthesis'. What this does is it regenerates ten percent of your health every turn.
To activate this trick, use Munch on the cactus enemies until you get Photosynthesis, but don't be surprised if it doesn't work on the very first try. You may need to Munch on a few cacti to get it. You'll know when it works because you'll see green numbers based on your current health. For example, you have one hundred hit points, so you'll see a green 10 every step you take.
When this works, run until you're totally out of EN, and you'll never run out of health because of the Photosynthesis. Just keep walking and walking and walking in a large area, ideally holding down the circle button with zero enemies in the area.
The combination of your newly gained Photosynthesis from Munch will combine perfectly with your Subconscious and allow you infinite experience that is limited only by your own patience. Keep going from one side of the area to the other and you'll gain EXP with every single step. It takes a helluva long time to hit the highest levels, but when you're able to gain plenty, which in the long run will give you plenty of skills.
It all depends on how long you're willing to trudge up and down the same section of map. A stubborn Ranger should be able to gain ten or twenty levels on a single map. Anyone more stubborn than that, see just how far you can go with this.