Assassins Creed 4 Black Flag – How to Achieve High Percentage Completion Without Optional Mission Goals

The agile guards use throwing knives as weapon, of which you can only carry 1 at a time. If you drop their weapon to the ground, it's most likely you already carry a throwing knife. If that's the case, just kill the guard with the knife you already have.

The best way to finish the challenge is by doing the following:

• Equip your fists (unarmed) and the throwing knife (it doesn't matter if you've got one or not)
• Initiate combat with an agile guard (they are the ones that don't wear hats)
• Wait for them to attack, then disarm them by pressing counter followed by break defense.
• Press shoot to throw your knife, killing the guard in the process.
• Rinse and repeat.

By disarming they mean taking the guards weapon and ultimately killing them with it. This can be done by disarming them with bare-hands equipped. When you counter and disarm, with bare-hands, Kenway will take the guards weapon and then you can kill them with their own weapon(at least that's what I prefer to do).