The video below of Clarky1005 shows a good spot to do this.
I recommand doing this after finishing the story!
I personally used Finn (fire spells) and Gen (stuns).
I killed one in turn one, and stunned another one. This way you can focus on the one that is still alive and not stunned. Kill him and make sure to cancel his attack when it is his turn.
Keep the third one stunned every turn to finish him off as last.
I hope this helped, good luck!