This can be achieved using weapons but not Vigors while riding a Sky-Line. To increase your chances of landing a successful shot, you can hold back on as this will slow you down to give you better aim. You can also zoom while on a Sky-Line to make landing shots easier.
If you are at a checkpoint before a Sky-Line and have the RPG equipped, you can grind out ills much easier as the RPG has splash damage and reloading checkpoints keeps your total running.
Refer to "Bolt From the Blue" for more info on how to grind this using the headshot method.