I got this one while doing the first mission of the Warrens, Stranded. After going past the blockade, you'll see 3 (sometimes 4) enemies in the water. Just zap an area near each one and they'll be fried, afterwards jump into the water to die and repeat after the game reloads. Kill 50 of them and this trophy is yours.
There are obviously other spots where this can be done, this is just how I got it.