You don’t have to physically complete the course in less than 20 seconds; your accuracy bonus will be subtracted from your time to give you a final time. Use the gun you’re told to use for the first 3 targets, then switch to your pistol to improve your accuracy. The only time you shouldn’t be sprinting is when you are shooting the targets. NEVER try to reload, it slows you down; always switch weapons. You have as many attempts as you wish, so go through the course slowly until you have memorized where you have to throw flash grenades, as well as where targets pop up, and you should have this achievement in a few attempts.