Knock 19 bikers off their bikes with the Phoenix's Dodge.

You can unlock this while attempting "Bytesize Takedown" so see it's description for how to effectively grind these out. To dodge with the Phoenix you need to press 'Y' and the direction you want to dodge in. With the specific convoy detailed in "Bytesize Takedown" there will always be either 3 or 4 motorbikes accompanying the convoy and they will always be at the back, so when you first reach the convoy make a point to stay close to the motorbikes, then when you're lined up next to one of them, dodge into them to knock them off. It usually takes only 1 dodge, though sometimes they seemed to survive the initial dodge and took 2 to knock off.