A Thunder Drop can be performed by jumping down from a high place, then pressing mid air. But you probably already knew that.
You can earn this trophy with ease during the side mission, “Masquerade”. You’ll be tasked in heading to the nearby park, where a bunch of citizens will be posing as statues in which you’ll have to eliminate. Head towards them and once they start attacking, climb up onto a nearby palm tree or the statue in the middle of the park and wait till they gather in one place. Jump and float towards the group, then perform a Thunder Drop on them. You will easily hit 7+ more enemies, as well as unlocking the trophy, along with the Discerning Taste trophy.
See Discerning Taste for a video.