Bastion's
Configuration: Tank ultimate ability allows him to convert into a tank with a lot of health and explosive shell ammo.
We will need to kill 4 enemies in 1 activation of the ability. Also long as the elimination comes up on screen, it will count, you don't need to deal the final blow or get the majority of the damage. Just use brute force to kill a large group of enemies.
I recommend going for this while defending the payload and having multiple enemies on the objective at the same time. It can also be done in any "capture" point that has many enemies, both in attack/defense.
If you can get a total of 4 kills at once, you will unlock the achievement.
If you'd like to learn more about Bastion, you can view this video. The achievement is described and shown at 11:02: