There really isnt a “set way” to level in black desert mobile. You can gain good exp from boss rushes if you have the stamps,you can do high ancient ruins over and over if you have the tablets. The rule of thumb is quantity over quality,you might think what do you mean? The higher level the monster the better the exp right? Yes and no,your combat power dictates pretty much where you can farm for levels. You want to be able to kill groups of mobs faster than anything. The difference between the level of the mob and the character affects the resulting drops and experience,For instance if you look at your world map and switch over to the monster tab you will see different colored enemies.
|Level Difference||Mob Color||Exp and Drops|
+7 or more
|+4 – +6||Orange||200%|
|-3 – +3||White||100%|
|-6 – -4||Green||100%|
|-7 and less||Gray||0%|
So to farm efficiently you want to be farming the orange enemies,while they are easier to kill they give the same amount of EXP and Drops as the red enemies. This is also dependent on your combat power,if you can kill higher level reds just as fast as you can orange,then definitely go for the reds.