If you really want it, just team up with someone who is really good as Zarya and time her ult right. That's how I got mine. I also got the other zenyatta achievement 3 days prior to that one by using it when the zarya on the other team used her ult and so did everyone else at the same time. I've been maiming zenyatta since I started playing and I know it's not easy. But you can't really boost no this game especially since you can only get achivements like that in quick or competitive. But I know that desperate feeling.