For this you need to you defeat Shellshock in the challenge Shellshock Returns without taking any damage.
Easiest weapons to use are the Harbinger/Mega Supernova and the Mini Turret Launcher/Mega Quasar Turret Launcher. As Shellshock reaches the floor fire the Harbinger when he retreats, a few DZ Cycloids will spawn. Use the harbinger again and the Mini Turret to dispatch them easily.
As always play on Couch Potato.