Go to Planet Arcterra. Then, go to the Sic Transit and look for a door with an orange force field. Go through the door using the Magmaul and you will be in a cave with enemies and a force field blocking your way to the Imperialist. Destroy all the enemies in the cave and the force field will disappear. Next, go to where the force field was and you will be attacked by an Artic Spawn. Defeat the Artic Spawn and the Imperialist will appear next to a door with a red force field. Get the Imperialist and you will have that weapon.