This can be got in single player, playing as Darth Vader and Kylo. I did it on the Resistance level, but any should work.
Swap to Darth and try and kill Kylo. It can be a little hard to land a direct hit but I found Vader's jump and light saber down attack to work best. It may take a few goes to get it right but it will work.
Some guides (for the x360) have suggested that you can get this only in two player mode with another player signed on to the second controller, but I found this not to be the case.