Low level - Go to Thieve's forest and kill the first few thieves until you see them start to run away. From there a big troll will come out and shoot out smaller ones, attack these guys and take out as many as you can and avoid hitting the bigger one. The longer he's out, the more little guys you get.

Mid level - When you are mid leveled (25-40) you will want to go to either Lava World, or Marsh. In Lava World follow the walkway until you get to a metal wall, the metal wall can only be broken when you use a sandwich so just keep storing up on those and killing some Fire Demons racking up quite a bit of xp. (note: they never stop coming up to the point where you break that wall. Keep it closed and you have yourself an endless amount of them coming your way.) In Marsh near the end there will be another troll with 2 Druidic Knights, continue the same as the first troll in Thieve's Forest.

High level - If you are over 40 you probably wont be going up to fast if you use the first two strategies. Instead you are going to want to go to the last level with the four closed off bosses. The painter is the easiest one to get xp. off of, he doesn't attack you until half his life bar is drained, take advantage and while hes painting use your strongest attacks on him, take as much life as you can before the horde of paintings attacks you.