Battleborn

PVP and PVE works. You can do this alone on private PVE match with full enemy and full own Team (makes easier). Meltdown with Oskar Mike worked fine for me.

This can only be done in PVP mode, and requires at least 2 enemy Battleborn on the opposite team. Set up a boosting session with friends, creating a private match and inviting them in. Meet in a designated area, damage both to a near death state, then finish them off a few seconds apart from each other. Characters with high damage output such as Montana would be good for this. Repeat 4 more times to unlock this.