There are two opportunities to unlock this trophy in the campaign.
The first is in chapter one right before rappelling down. There is a small herd of these creatures to the right of the freeway. Open fire to kill them.
If you miss killing one in the first chapter, there is another chance in chapter eleven. Make the choice to save a bullet in the very beginning and proceed forward.
There will be a single oryx eating a dead body. Aim carefully and lay it out. Make sure not to miss as you only have a single bullet at this point.