drill sergeant

1. A non-commissioned officer in military or police forces usually assigned the duty of training new recruits. (More formally called a "drill instructor," as the title of "sergeant" is only given in certain territories and branches.) My drill sergeant made me run laps around the barracks until I vomited. Your drill sergeant may be tough, but they are making sure you become a reliable soldier. The troops abruptly stopped marching when the drill sergeant called them to attention.

2. By extension, someone in a position of authority or mentorship who is particularly exacting or ruthless. Our band teacher is a real drill sergeant, making us repeat the same sections over and over again until they're perfect. The coach may be seen as a drill sergeant by the athletes, but the success she has achieved with them is undeniable. You'll like working for Matt. He expects results, but he's not a drill sergeant about it.