If you want to gain some levels without much effort and are around a 25 to 40 level range, try going the forest levels under the hard difficulty setting. You can usually get about 8,000 experience points from it.
Find someone who has played the game longer than you have. Ask them if they can give you a high powered Mag. With this Mag, you can start using weapons from the Ruins (if it is at a high enough level). Also, you can join in a multi-player effort. If you are a Ranger, you will have an easier time then a Hunter. When a teammate has weakened a monster, aim and go for the kill shot. You should move up levels easily.
Since Hunters are naturally stronger than Forces and Rangers, they have an advantage. To gain levels quicker, defeat the Caves 3 boss. Hunters can use an effective Sword or Partisan against him by hitting all of his parts. After that, talk to the Priciple and go to Mines 1. You can now do a great deal of damage on the Mine 1 enemies. They are weak. The only three to watch out for are Sinow Gold, Sinow Beat, and Granas. The basic enemies in the mines area give about 15 to 22 experience points. By the time you get to Mines 2, you should have already gained a couple of levels. Mines 2 also offer a great deal of experience.
Go to the Forest (under the hard, very hard, or ultimate difficulty settings depending on your level). Enter a room which has a Monest (or Mothvist in ultimate) and killall the Mosquitoes without killing the nest. Once the nest stops spitting out mosquitoes, return to town via Telepipe or Ryuker. Return. The nest will spit out more mosquitoes. The experience given varies depending on difficulty, but the ultimate mode mosquitoes give about 230 experience points each.
Play in multi-player mode with a group of either high levels or low levels. Play through the game as you would in solo mode. Many of the monsters are harder to defeat, and you will get five points extra experience per how many people are on your team. By the time you complete the game you should be at level 20 or higher, depending how many monsters you have killed. Note: When you make it to hard mode, you will start finding special weapons earlier because your luck points are being evaluated as a team and not individually. Then, when you play in solo mode you will be able to pass through it without problems.
Use a weapon that can confuse an enemy. Use it on a Honest and when the little fly things appear, they will be confused and will kill each other giving you the experience.
When you fight a Pofuilly Slime, use Barta or another Ice technique on it. The enemy will duplicate into two. You can then make a copy of the copy (you can only make one copy of the original). You can create a total of six Pofuilly Slime. The copies will give you double the experience of a normal one.