This achievement is selft explationary, if you are going to reach lvl 50 you will unlock this achievement on your way, but here is a method (CO-OP only) where you can boost lvl on a specific lvl with a overpowered partner.
So he helped me to reach lvl 50, his gamertag is RE6ORCPC (here is his profile
RE6ORCPC ) and he always want to help people on this game, because he is overpowered, he have almost all best upgrades and equipement of the raid mode on the 360 version. If you want to lvl up or do the ghost ship talk to him.
He made this video to show how to lvl up fastly with him: