The Warper npc that you can find in every town can warp you to leveling areas all over the map. You can choose the map based on your level. If you are strong enough to go to the map above your level, you are free to do so.
|1-20||Prontera Field 8||Poring, Little Poring, Lunatic, Fabre|
|20-40||Payon Dungeon 1||Skeleton, Zombie, Familiar, Poporing|
|40-70||Orc Dungeon 1||Orc Zombie, Orc Skeleton, Steel Chonchon, Drainliar, Familiar|
|70-90||Glast Heim St Abbey||Evil Druid, Wraith, Mimic|
|90-100||Glast Heim Churchyard||Dark Priest, Evil Druid, Wraith, Mimic, Hunter Fly|
|100-110||Veins Field 3||Magmaring, Drosera|
|110-120||Ruins of Juperos 1||Venatu, Dimik|
|120-130||Rachel Sanctuary 1||Vanberk, Isilla, Hodremlin, Beholder|
|130-150||Dicastes Dungeon 1||One-Horned Scaraba, Two-Horned Scaraba|
|150-160||Bifrost Tower F1||Cenere, Antique Book|
Mercenaries can be very useful when it comes to leveling, especially when you're a newbie and don't have good equipment yet. By the time you graduate from Limit Academy, you will have a few random Mercenaries in your inventory, but the good ones come from Mercenary Manager in Limit Group, you can hire Grade 7 Swordman Mercenary (Lancer).For this method you will also need some Fly wings that you can buy from Tool Dealer.
Take your Mercenary to Rachel Field 01, and use the Fly Wings to teleport and find Desert Wolves. DO NOT MOVE or cast any skills, all you need to do is just use Fly Wings and watch your Mercenary kills the Desert Wolves. If you move, you will no longer invisible to those Desert Wolves.
When you first entered the world of Limit RO, after the Academy, you will be in Limit Group Headquarter. You can join Limit Group and do the Bountyd Quests. These Boards come with different level categories, you need to choose one that fits your current level. Beside EXP, you can also get Limit Group Coins and Limit Merit Badge as a reward.
Some instances do give a lot of EXP, mostly by killing the bosses inside. The most popular one, is of course, Infinity Space since this instance's main purpose is for leveling. Endless Tower also gives a lot of EXP due to the numbers of monsters and bosses inside.