Difficulty: Easyish
I find this easiest with the 3 barrelled shotgun, which you get later in the game, but the normal one should work too.
On the first mission, when the executioner boss appears, this is your chance.
Run past the bus and onto the left building roof. Jump onto the bus and wait for a Majini to catch up.
When your opponent is about to jump across you need to aim above their head slightly and fire. If you got a headshot and killed the opponent (which is likely with a shotgun) then there's your trophy. If not, then try again as the enemies keep spawning.