You'll pop this Achievement once you complete the 9th assigned mission of the game. I played quite a bit before doing more than a couple assigned missions and did not level up my hunter rank at all. From what i've read, any xp you've gained before completing the final assigned mission is awarded after.
*** Spoiler - click to reveal ***The Encroaching Anjanath is the 9th mission, requiring you to slay the Anjanath within 50 minutes and without fainting more than 3 times. Here is a video of me completing it using the Iron assault III. I had already researched the Anjanath enough to be able to track it but i showed how/where to stop to level up if you need to.