This is probably the second hardest achievement in Overwatch. (I felt using speed managing wall riding and finding weak targets and surviving in Lucio's Floor is lava achievement was harder.) I wanted to put down some things I learned about the achievement.
How to best setup to achieve it-
1. The discord orb has to be on all 4 targets when they die.
2. A shot has to land on the target while discorded.
3. The six second window is exactly 6 seconds starting from the first death, not shot.
4. Headshots significantly increase the likelihood of completing the achievement.
5. King of the hill and 2 CP maps such as nepal, lijiang, Hanamura, Anubis, and Volskaya felt like the best chance maps to complete it.
6. Follow around your dps. Try and focus the same targets they are looking at.
7. Consider alternate binding of the discord orb to your mouse.
8. Prioritize setting up for an engagement before it happens and surviving.
I really want to stress these last 3 points. With the floor is lava, It felt like you get a lot following around the tanks, but in contrast Zenyatta did not feel this way. Almost every time I was close I found that it was working with a dps player, the action of spamming the discord with e, limited what movement i could do to survive with wasd character movement. Thinking about how the engagement plays out and which targets to prioritize helps a lot to lead into the effect of rolling the other team. Ana's stim on a lucio did not appear to be as effective in my opinion as lucio's speed boost. If you tag a target with the orb then move on to the next with 2 friends gunning them down, it did not seem to give the achievement. I'm glad I grinded out the achievements as they helped me to understand how the game works. I hope this helps and wish you all the best of luck.