The only way to take out important types, ie, high ranking officers, is an insider attack.
To get near them, you have to penetrate the front line, the rear lines, the forward bases, the rear bases, then however many kilometres it is to headquarters or base camp.
And even if you use a 'quiet' airrifle to kill a General, that shit will be all over the media and all throughout the military, that a General has been assassinated. Which kind of negates the idea of quiet killing.
And a helmet will stop a air rifle pellet. As will a glass window with a fly screen.