General information on the mission
The fortieth mission in Metal Gear Solid V is a replayable one. Its requirements are nearly identical with the ones of the fifteenth one, i.e. the duel with Quiet. The walkthrough for it has been provided there. Unfortunately, this time around, defeating Quiet is FAR MORE difficult, because your character, due to his poor armoring, can be killed in one sniper rifle shot and additionally, the tactic of dropping supply crates on her head does not help. This is not it, because you cannot use the Reflex Mode nor the Chicken Hat, here.
The next page includes details o defeating Quiet, subject to the Extreme rules.