In the mission called "The Instructors" your task is to kill three instructors. They are in a training base where you can find many training soldiers. Avoid open conflicts to complete the mission.
First, circle around the base so you can be on the north of it. There, you can find electric power generator. Approach it and deactivate. This will shut down the interrupting device which didn't allow you to use a drone. Also, if there are problems, the enemies can't call for support.
The instructors are in a place that can be accessed by killing only a few enemies. Identify them by using a drone.
When you eliminate two of them, they are in the bigger part of the base, go back to where the base begins and take care of the last instructor.
When you kill him, the mission is over.