Have you tried hunting down the silvers? I have only ever seen two but they are randomized armor elites that naturally spawn and rarely try to kill you. There are two in the swordbase level if you need proof. One is outside of the complex and runs into the deserted, left side of the complex after you have completed the beginning of the mission and you are fighting your way back into the base. He is found at the top of the hill after the walk up the place where they try to prevent you from bringing your vehicle in. The other one I've found is in the base. He can be found on the floor below the general when you are fighting your way up to the place where you shoot down the banshees. He will be firing two plasma rifles and dodging around but otherwise staying relatively still. I can confirm that these things spawn when you play normal mode but I don't know about heroic or legendary or easy. Edit: inb4 "Those are just ultras" These elites often spawn with more high level armor but with only silver coloring.