This achievement is earned by completing story missions and Rebel activities earning XP. Rebel missions and Story missions will earn you XP to level your character up. I was about 50 percent done through the game when I hit top rank.
Something to keep in mind, you will get this achievement when you hit rank 30; however you do not need to upgrade to Tier 1to get it. Once you get to level 30, the game gets more and more difficult with each mission.
It is your choice to upgrade to Tier 1, and the difficulty does get harder. Each NPC takes a lot more shots to take down, and stealth is a must to complete most missions