Simple enough, beat yet another Master. Although this one is slightly different as he is IMMORTAL.
So, the initiation fight is Zamasu and Goku Black... just defeat Black then survive until the time runs out.
Now.. Lesson 1 and 2 are new concepts (for masters, anyway) as you must press the buttons that appear onscreen. This can be a pain as sometimes the buttons won't register but keep at it!
Final lesson, you have to defeat Zamasu with his ultimate. This will be no problem for a Strike build (most likely one shot him) but for blast builds, just spam the attack until he dies (his health regenerates so be aggressive)
The DLC Achievements bug has been fixed as of 27th January 2018, so go ahead and get it popped!