This Achievement unlocks by reaching Mastery Rank 5 as any of the 6 Classes in the base game
There are 5 Mastery Ranks per Class, with 3 individual Challenges to complete for each of them (a total of 15 Challenges)
Progress can only be made to the Challenges of the Rank you are trying to reach, e.g. you cannot make progress towards the Challenges found in rank 4 if you are Rank 2
Progress updates once at the end of every match. Due to being able to change Classes within a match if losing a Round, it is possible to make progress towards Mastery Challenges for 2 Classes in a single match
When completing all 3 Challenges within a Rank, your Rank increases and the next Ranks Challenges become available to complete. You will also receive 3 new Kits that can be equipped, branded with the Class and Mastery Rank of what you have reached (these are found in Customise)
Here's a guide to all 5 Mastery Ranks as Thief:
Here's a guide to all 5 Mastery Ranks as Blade:
Other Classes are yet to have guides made