What weapons count for the “kill an enethe with every type of weapon” Abstergo challenge

The white skull icon seems to indicate which enemy is engaging in combat with a crewmate that needs saving and is accompanied by a blue dot over said crewmate in trouble, this skull is to help indicate which enemy to focus on to help save that crewmate.

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.