Easy experience, money and skill points
Go to the Obel Ruins. Enter the first floor repeatedly until you face six Flying Lizards and three Skeletons. After killing the lizards, you can box in the three Skeletons and kill them repeatedly, as they keep regenerating. Use a controller with an auto-fire feature and keep X held. After a few hours you should have no more problems. Make sure to use characters who will not die in one hit, as every now and then they may actually get a successful attack. Any healing that is needed is usually covered in the leveling up, but you can use Element Beads or Runes to charge the area under your fighter to give them healing every round, as well as a stat bonus.

Easy experience
As long as you do something on your turn, the character gains experience. Go to the ruins of Obel. Do not get the 6 bat and 3 skeleton map on the first floor. Finish the enemies off then gather around your highest leveled character. Use Medicine or a skill (such as Gary's Backup) on the higher level character. When you gain the ability to switch in and out, you can ensure that everyone takes advantage of this. Using a Medicine on a character just one level higher than you gains 130 experience points. The experience gained is higher if the character has a few levels on you. Once everyone is at the same level, go to Callonball and "hunt monsters". There is usually a group of monsters that is two or three levels higher than you. Once you get a character one or two levels higher than everyone else, proceed back to Obel and repeat the Medicine/skills trick. This is cost effective due to the fact that 99 Medicines cost 1980 Potch, and at this point you should be making 5,000 Potch or more per battle.

Completion bonuses
Successfully complete the game and save when prompted. Load your cleared game file to start the game again with all previous skills, items, and money. Note: You will lose your runes. You can also press Start during intermission sequences to skip them.

Suikoden 4 references
Load a Suikoden 4 saved game file with 108 stars when prompted when starting a new game. This will unlock additional dialogue and two Suikoden 4 related characters (Hero IV and Snowe).

To find Hero IV, return to the Island Nations just before the event at the Laboratory on Kooluk continent. Go to Obel then speak to Lino under "Gossip". Go to the deserted island, travel to the coast, then help Hero IV defeat some monsters. Hero IV will have the name given to him on your Suikoden 4 saved game file.

To find Snowe, first get Hero IV in your party then go to Razril. Talk to the little boy under "Gossip". Help Snowe defeat the Furballs in the back alley.