This isn't as bad as it sounds. For this trophy I would have friend or another person tag along whom you can communicate with. You don't actually have to do any damage as the one you empower will get you the assists for the trophy, but it doesn't hurt to help out.
Have that friend choose a character whose ultimate kills instantaneously and in rapid succession. So characters like Reaper, Pharah, or McCree would be a good choice, but I found Reaper to be the easiest. Both of you should build up your ultimates, then wait for the majority of the enemy team to gather up, power your partner up and have them use their ability . Doing this during hectic moments in control, capture point or at the end of payload work very well.
If you are doing this by yourself, I would suggest waiting for the ends of each match type as people tend to gather rapidly and allies use their ultimates more.