Here is how I unlocked it - not sure if this is the 'correct way'.
When you first encounter Onmoraki, your NPC chums are distracting the enemies on the ground.
After a while Onmoraki appeared on the roof of a nearby building - it looked like background stuff until it's projectiles started hitting me!
I shot the big glowing horn about 3 times with my Matchlock and it 'fell off', Onomoraki then screamed in anguish and flew off.
Trophy unlocked during it's flight.
Try it - might work for you as well!