Easiest way to get this:
Head over to the first enemy Outpost (that when you get the "Welcome to Kyrat"-trophy) and release the elephant (throw stones on it so it gets enraged). The elephant will kill all enemies and the trophy is yours.
Another solution:
Stealth as long as you sabotaged the bell, so it doesnt work.
Now you can all enemies - no matter if they see you, because they cant ring the bell.
After all enemies are down, the trophy unlocks.