To get this achievement you'll need to reach rank one in each infantry class. Assault, medic, support and scout classes. Assault and Support you will need 50,000 points each to reach rank 1. Medic you will need 45,000. Scout you'll need 30,000 points.
You earn points for your class simply by playing your class. So play the objective, help your squad and earn medals. Below is a video showing how to view your rank in main menu and in a match.