Here is a list of the monsters you should be fighting at each specific level. The stronger the monster you kill, the more experience points you get. Killing the appropriate monsters for your level makes it easier and faster to level up. Here's a training guide for those of you who wish to do so:
Level 1-4: Kooii & Bramble Donguri: These are located right outside of town, in either Woody-Weedy Forest or Woody-Wordy Forest depending on your faction. These monsters won't attack you unless you attack them first.
Level 3-6: Leaf Boar and Angry Leaf Boar: These are at the southern part of the Woody-Weedy Forest/Woody-Wordy Forest. They are aggressive, faster and stronger than the Kooii, so be careful. Also, be sure to run away if you see BULLDOZER, and definitely if you see BULLDOZER'S BROTHER. You may be able to kill Bulldozer Jr. They give a nice Canine Ring.
Level 6-7: Flower Elemental: These are flower-like monsters who will not attack you unless you attack them. They can't move around so you have the advantage of escaping them when your HP is low. They are ranged attackers. They also give you some pretty good drops such as Old Feathers, Double Shot III, Wooden Staffs, Blunt Swords, and enchantment scrolls.
Level 7-8: Swamp Mushrooms and Swamp Boars: Swamp Mushrooms are fast and give nice drops such as Vol. III Skill Books and Small Treasure Boxes. They’re found at the northern part of Mushroom Marshland. Swamp Boars are a level higher than Swamp Mushrooms, and are located at the center of the Mushroom Marshland.
Level 8-10: Swamp Spiders, Poisonous Spiders, White Spiders: White spiders do lots of damage. These spiders are pretty good for leveling from 8 to 9, and give some good drops for trading quests such as damaged pamphlets and old feathers.
Level 9-11: Red Crabs: Red crabs are good for leveling from 9 to 11. They give nice equips, but have low drop rates (like lvl 12 Mittens). They also drop Pirate Coins which can be traded in for 300 EXP and some Gold, after lvl 11. Watch out for Woopas / Roopas when hunting these as they will attack if you get too close. I don't recommend crabs for level 9 warriors, unless you have at least 15 armour.
Level 11-12: Sandmen: Sandmen give good experience from level 11 to 12. You can also train here at level 10, but they are a bit harder to fight. They drop Pirate Coins and occasionally Mystic Stones and Red Coral.
Level 13-15: Roopas and Woopas, Continent Bulldozers, & Crows/Fox, & Mature Fox: Roopa and Woopa are the hardest unnamed monsters on the beginner’s island. These fish are hard, fast, and can attack in huge groups. Continent Bulldozers are found in Lanos plains/Foxes are found in the Forest of Grave. When you reach level 14, kill Crows/Mature foxes.
Level 15-18: Testing Woopas, Marsh Donguris and Skeletons: Testing Woopa are slow and have good drops such as Weapon and Armor Enchant Scroll C. Marsh Donguri drop Donguri Leaves which can be used make a Donguri Leaf Hat. Skeletons are good to kill for Bone Armor and Pieces of Bone.
Level 18-20: Magma/Mutant Gosumi, Ghost, Snaketail Lizard, Beholder and Drunken Kooii: The majority of Light House monsters have good drops. Kataru Mountain/Forest with Ruins monsters have less drops and seem to be less likely to drop any enchants. But if you don't prefer the risk of being pked, Kataru Mountains/Forest with Ruins is the place to be.
Level 20-21: Magma/Mutant Gosumi, Big Boar/Fire Bird: This depends on your willingness to grind. You can kill Mutant Gosumi/Magma if you have the time for 0.09% a kill. You will need to have a scrolled up weapon for the Boars, or you'll miss a lot. Or just kill Fire birds if your from Siras.The LightHouse 3rd Floor Dark Beholder gives ~0.11% a kill and are the easiest based on cost (time/difficulty) benefit. There's something to be said about Drunken Kooii at 0.07% a kill. Waste of time killing Beholder and Skeleton now since it's 0.01% a kill and Skeleton are worthless except for farming. The Pirate Ship is ok too, depending on the animus of the other faction and about the same value on each kill as in the Forest with Ruins / Kataru Mountains.
Level 24-28: Cactus bees / Desert gosumis: Bees and Gosumis give good xp until lvl26-27 so you can also try dongs if you got a lot of hit-rate. Ask for a party with a lurer (mage or warior) and 2 rangers but rangers can also lure bees cuz they're very slow. And if u are in a good guild you could get free lure ;)
Level 23-25: Desert Gosumi, Desert Sandmen / Siras should kill Desert Gosumi, or Desert Sandmen. The gosumi and sandmen drop Old Scrolls (which when u you double click it in the menu you get either recall scrolls or D, C, B, or an enchantment scroll) and other items such as thick leather that you can use to make armor and helmets. You should try to get a lure or a noob mage party so its easier to kill and the noob mage gets some nice little exp ;)
Level 26-29: This depends... If you are willing to risk dying (get Guardianship of Siras / Protection of Lanos from Secret Vendors to prevent losing that precious 1% when you die). If you have the money to get the Guardianship/Protection, then you could go and kill Redeye Gosumi / Sunset Flower (lower exp), desert sandmen / Doleys' (somewhat better), desert beholder / sand donguris, (They all drop the same items, but beholders and dongs seems to drop more items), or 3eyelizard / Desert Ambush (being the hardest). SHAAACK is the desert valley and BSSSZSSS in the arid grassland, that you might need to look out for. They can attack you when you are inside the glitch so either avoid him or kill him. (They're level 40 so look out.)