The Fast Draw skillshot is achieved by killing an enemy with the Screamer as soon as he is in your sights. Aiming for the head will give you a better chance of earning this skillshot. You can also dwindle down an enemy's health with the PMC. Once you think he's been weakened enough, look away from him, switch to your Screamer, aim back at him, and immediately fire away. If your estimation was correct, he should die in one shot. This strategy can also be done to earn the Snapshot skillshot.