To unlock the Buzzkill trophy in Far Cry New Dawn, you will need to kill a total of 10 brawlers with a saw launcher.
Brawlers are the big dudes with shields. There are a couple of good places to farm the kills needed for the trophy. The best spot I found was at The Bonecrusher Pit in the centre of the map. When you arrive there, there will always be around 5-8 Brawlers in the area. Equip a saw launcher, and fire at their feet when they approach and they'll die quickly depending on their rank.
Once you kill 10, the trophy will unlock.