Easy was to do this is with Armin in any mission that allows you to play as him. Since Armin's ability to to command people you can lock onto a titan and have all your allies kill all 4 limbs plus the nape of the neck. So the absolute easiest way to get this is once you unlock Armin as a playable character in the Survey Missions choose one of the easiest ones from the first menu and just rinse and repeat.
Having people in your group use B while playing as Armin and you can command your teammates to attack a certain limb. Hope this helps.