Ahoy Maties! This achievement tasks you with obtaining level 50 in the Sea Dogs Alliance.
Keep in mind simply leveling up will not get you the achievement. After reaching level 50 you must purchase the corresponding promotions from DeMarco in the Arena.
Getting to level 50 will take time as a win will net you 4800 gold/exp, which isn’t a lot once you get into the 40s; however, you can use Doubloons to purchase additional levels (but I wouldn’t recommend doing that until you have at least played your 200 matches and got your 100 wins). With the reaper chests now a permanent addition you have a consistent way to get doubloons if you wish to use this method to rank up the alliance.
Hope you find the info helpful! See you on the seas!