They are extremely easy kills with the argentavis. Pick one up, fly up to a safe location in the city with a flat surface beneath you and claw away at its health. It will still hit you every so often, but you won’t need to worry about scouts or defensive units coming for you.
Usually, they drop blueprints +/- 5 levels of the enforcer you kill.
To get the crystal and element dust you’ll need, use that same argentavis to either break lamp posts and lights throughout the city (it will loot automatically) OR better yet, carry your doedic and let it do all the work on those lamps. They’re the most efficient for collecting element dust.