Instructions
Find the most powerful creatures and highest-level quests you are able to complete. All characters should fight monsters and complete quests, but if you're at a lower level and go after monsters that are a little bit more powerful, the experience-points (XP) payout will do you much more good. Additionally, completing quests often comes with an XP bonus, so the more quests you complete the faster your level will raise up.
Link up with other adventuring parties. Killing monsters on your own is one thing, but if you join an adventuring party then you can bring a bigger force to bear and go after larger creatures that are worth more XP and treasure. And grinding never seems quite as tedious when you're part of a bigger group.
Purchase 2X cards from the online store. These cards, which have to be purchased with real money, will allow you to shoot up your XP a lot faster. You can also find clerics and other classes -- that have spells that allow you to gain more XP from killing monsters -- to follow you around as a two-person party for a similar effect.