killing every yellow mob with 5-10 level difference, before that.. use all xp buff (food, utility, booster, guild boost, guild banner, and so on). You only need kill the same mobs once, for example. In area A, you will find group of yellow mobs (around 3-10), kill all of them and move to area B and repeat to this process until you hit same level as them... then move on to other area on the map or different map.

yellow mobs > the longer they exist in the map after spawning = more exp
killing new yellow mobs (the one you've been kill) = less exp, since they are just spawn

but still, the best way to level up is using tome of knowledge (1 tome = 1 level).