This is story-based and can't be missed.
After the missions with Zorah Magdoros have concluded, the game switches over to High Rank missions and monsters. It's the same idea as before: Grind to get new gear, which lets you hunt new monsters, which lets you grind new gear.
Very soon after completing the achievement for seven-star assignments, you'll be tasked with tracking down Nergigante and eventually hunting him. Once you complete the mission A Wound And A Thirst, you'll unlock this achievement.