To add to this: Bonus XP (whether from event bonus, potions, Lesson, etc) has no effect on evolving XP. The only things that matter are the level of the mob relative to your own, and the number of people (or mercs) in the group.

So the ideal leveling speed is the highest level mobs you can kill the quickest. Find the balance there and grind away. This is why GMM tends to be favored. High level mobs that die quickly (and aren't too much of a threat as far as tanking requirements go).