To unlock the "Bolt from the Blue" achievement in BioShock: Infinite, you will need to kill 5 enemies with a head shot while riding a Sky-Line. Sky-Lines are riddled throughout the game, so there is no need to go searching for one.
What this achievement really comes down to is aim and weapon choice. For close-range combat, you might want to opt for the shotgun or burst gun, but for longer distance and more gangster head shot, there is nothing better than a sniper rifle.
Check out the video guide below for weapon options and gameplay.
Click here for more achievements/trophies for BioShock: Infinite.