This is best done with an AOE/caster class. In FOTD, there is the alpha wolf boss where there is a large rock you can get up onto above him. Climb up onto that rock.
Hit the boss with a spell. You don't want to kill him. Everytime you hit him, he spawns several adds... keep hitting him until you have as many adds as you'd like to kill with aoe!
Because neither the adds nor the boss can get to you, the adds just keep spawning w/o attacking. It still works.
You can level up from lvl 40 to around lvl 50 using this method overnight.