The highest possible level in the beta is 13, and that’s a hard cap; it’s not that you just gain exp every slowly once you get to 13, it’s that you gain zero exp from then on. This is apparently something set specifically for the Beta, and isn’t due to very harsh diminishing returns. In any event, 13 is plenty high enough to devastate the monsters, as several videos have demonstrated.
This first video, recorded by MrTidus, shows a kind of “greatest hits” of a Clvl 13 Monk, as he massacres everything in sight in a bunch of tightly-edited clips. There’s not much variety in skills, with the combos Deadly Reach and Crippling Wave taking out everything with a single hit, and heavy use of Dashing Strike to essentially teleport the Monk from group to group without a wasted second.
Click through for a couple more videos showing an equal disregard for monster dignity or game difficulty, and the highest monster massacre scores you’re likely to see in the beta. (Over 100!)
Here we see two short videos of the same Clvl 13 Monk abusing the Jar of Souls quest and killing the skeletons almost as quickly as they can spawn. He racks up over 100 kill massacre bonuses both times.