Clarice Starling (The Silence of the Lambs)
Clarice is still just a student at the FBI Academy, but she coaxes information out of Hannibal Lecter (while admittedly making a stupid mistake and giving him personal information about herself) to track down serial killer Buffalo Bill, evades Bill in the basement while he has the benefit of night-vision goggles and manages to fatally shoot him, saving Catherine Martin from becoming a skin suit. I mentioned she’s still a student, right?