IIRC, agile guards are ones that don't wear a hat, and they tend to strike multiple times if they land a hit. They also need to have their guard broken or countered in order to land a hit.
You're going after the "disarm and kill agiles with their own weapon" challenge, right? This one tripped me up too, because what actually counts as their weapon is actually the throwing knife. So do a disarm (unarmed, circle then X) and then quickly switch to the throwing knife that you just grabbed from them and use it. --- Come see my game collection: http://www.backloggery.com/bakonbitz