Super easy and can be collected with normal gameplay. Look for groups of Rakk flying around in the sky, find a group that contains at least 5 and fire one shot into the group. That shouldn't kill any but it will attract their attention and cause them to fly to you to attack. When you feel they are close enough fire into the group using (preferably) a machine gun. Since the Rakk stay in a close formation, taking out 5 in a 10 second period should not be a problem.