Leveling to 40 quickly : KO Submissions

Always Hunt in a (Good) Party: you kill a monster by yourself, you get "x" experience points (xp). You kill the monster with a party member, you each get (roughly) 70% of "x" experience points, and so on and so forth as the party size increases. The game encourages partying. Also, party members will have complementary skills that will make you fight better. It is important to find a good party with an effective leader
Get HP Buffed Whenever Possible: every time you die you really delay your leveling. It helps tremendously to be hp-buffed. You can get buffed by a priest or by using a +hp scroll. You can buy 45 +500 hp scrolls from the PowerUp Store for 15 Knight Cash (KC), and 1 KC = $0.01 USD, so that is 3 cents per scroll. Being hp-buffed can often be the difference between dying (e.g. an annoying lag death) and surviving a fight
Get Someone to Power Level You: especially for levels lower than 20. A level 46 mage leveled me from 2 to 22 in an hour and 40 minutes. Make sure to do the Beginner's Quest (you get that chicken next to your name) from Selith in Moradon, so that higher level characters can party you. Also, have the power leveler invite other chickens to bring down the average level of the party, so that you'll get more xp per kill
Always Do the Level-Up Quests: these quests are available every tenth level (i.e. level 10, 20, 30, etc) and they are well worth your time. Non-rogue and mages-without-Chill may have a tough time soloing the level 30 Recon Quest. But rogues (by using Hide and Swift) and mages with Chill should be able to solo without difficulty as long as they are careful (i.e. run or Hide instead of risking getting hit). For the level 40 quest (Battlefield), you actually get more xp per kill if you party with someone else
Don't Get Fixated on Acquiring Items/Equipment: it will be much more effective to farm when you are higher level. You'll want to have a solid weapon to hunt with (e.g. +5 horn xbow for a 3x archer, a pair of +4 sword breakers for a 3x assasin), but don't get fixated on raising money to equip your character. Goblin armor is your best bet as far as affordable quality armor for any class. If you are a mage, don't sweat getting the level 35 scroll. The drop rate for those scrolls is pathetically low. Farming haungas in the hope of getting a scroll is an exercise in frustration and futility. When you are higher level you will be able to buy one or at least farm haungas much faster
Use Potions Liberally: don't rest to recover hit or mana points. Use potions (pots) instead. The pots usually pay for themselves because you'll rack up more kills in a period of time. For cost efficiency, I recommend sticking with the lower-cost pots (e.g. 240 mana, 180 hp). At level 40+ level, you'll probably want to switch to the higher hp pots, but I still stick to the cheap 240 pots because they are a better value
Don't Hunt on a Laggy Server: hunting with lag significantly increases the chance that you'll die, and you won't be able to kill as many monsters in a given period of time. If a server is laggy, check the others to see if they're better
Stay Out of the Abyss: some people feel it is the best place for low-level characters to farm (Abyss gems). I'm not going to debate that here. But the Abyss is not a good place for leveling, because it's too crowded, there isn't much room to manuever, and monsters spawn all over each room. Therefore, the risk of dying is much higher than hunting on the main nation map
Pay for Premium: (I listed this last because some people freak out when they read this) a lot of people refuse to pay K2 for Premium because of bugs and issues with the game. I'm not going to get into that here. But the reality is if you don't have Premium, it's going to take much longer to level. With Premium, you get 20% more xp and the death penalty is halved. Yeah, it costs $9.95 USD, but that's the price to see a movie at the theater or 2 trips to an unhealthy fast food restaurant
In terms of what to hunt (you can find these monsters on the maps for El Morad or Karus):

Levels 1 to 20: stay in Moradon, find a high-level character (you can generally tell by their attacks and their gear) and ask them to power level you
Levels 20-30: hunt werewolves, undying, and the lowest-level goblins (kobolds)
Levels 30-40: hunt meganthereons (megas), death knights (dks), mid-level goblins (bugbears and pookas). In the goblin village, be aware that goblins are fast and have a long attack reach. Goblin bouncers can be riskier to fight than megas and dks, because they can hit you even while you are running away with Swift