You want to complete Just Cause 4 with 100% and unlock all trophies? Then you also have to do the following achievement:
Turn 10 Black Hand into Cows using the Cow Gun in a single session
A complete overview of all trophies and achievements can be found here.
Cow Gun Location
Unlocking the trophy is easy, you just have to find the Cow Gun. This is located on a porch, at a small farm in Vaivenes.
You can not kill opponents with the Cow Gun, you just turn them into a cow. To unlock the trophy you have to do that 10 times.
If you have turned a soldier, he will continue to shoot you. You can simply drag the transformed soldier with your hook to a wall or ceiling, then he will not bother you anymore.