For those of us that had level 50 and enter this DLC (The Secret Armory of General Knoxx), this is an easy achievement. The DLC has some really high experience enemies to kill, I had this achievement by the northern most roadblock by the Crimson Lances. The drifters are worth alot of XP and will sit off in the distance. Their weak spot is the glowing sack hanging off their back (I like to think of it as their buttock) which can be sniped from afar since it hangs down a little.
Also note that if you had your XP bar maxed out for level 50 (and 1 XP point would have put you into 51) that your XP bar is now reset and you start as if you just obtained level 50.
Happy hunting!
Since the last title update, you can unlock this achievement sans the appropriate DLC.