Dungeons 3

This is done in the "Once Upon A Catapult" level. In this level you can capture 3 catapults scattered across the map. These catapults are extremely powerful and can kill most of the Heroes in 1 shot. There are plenty of Heroes to kill and the top 2 Hero camps endlessly spawn Heroes so there's no way to miss out on this trophy. Once you've killed 50 Heroes with catapult attacks, you'll get the trophy. To launch a catapult, hit then to highlight the catapult, hit then aim at a group of Heroes and press confirm, then wait for the lobbed shot to land.