50,000 lifetime mob kills means that you destroy 50,000 enemies total over all the games you play. All you have to do is keep playing more games and destroying more enemies and you will eventually get 50,000 mob kills.
This isn't 1D, it's 2D from the perspective of the 2D plane in question. We are just seeing cross sections of that plane, each one showing width (1D), and together all of them showing depth (2D).
 "ZunderFury"){kind=icon}
 "Z-Rox"){kind=icon}