The First Descendant

Mastery Rank is an overall progression level for every action you take in the game. You will get points for leveling up characters and weapons, unlocking new Descendants, and completing missions. You will want to vary your activities and the weapons you use to keep getting progress toward this. Once you earn enough Mastery XP to level up, you need to return to Albion and visit the Prime Hands terminal to the left of the spawn point to actually apply your next level, though if you fail to do so right away you'll still earn Mastery XP, so don't worry about quitting an activity right away to level up, just take care of it the next time you're in Albion. This achievement will come along the way to completing all the other achievements.