DRAGON BALL: Sparking! ZERO

Select the Goku Black Saga and go to the episode "Act Three Begins". You will need to be on default difficulty to unlock the branching path. The bonus objective you need to complete is to beat Goku and Vegeta before the match ends.

As usual you will want to get to sparking mode as soon as possible, get in close with the dragon dash (RT+A) and spam X, then use your ultimate (RT+B) when sparking is about to end. I like to start the match by charging to 3 bars straight away and using a rush attack (RT+Y) to get a fast start. If the rush misses, just restart the battle and try again so that you can maximise your time in the battle.

You can do the next episode, "Danger Posed by Zamasu", on easy difficulty.
However, on the next episode "Their Choice" you need to go back to default difficulty to complete another bonus objective. You will need to beat Goku before you fuse with Zamasu. The same tactics as before should work.

Then for the final episode "Proof of Justice" you can switch back to easier difficulty if you like and unlock the acheivment afterward.