The Silver Star, or SS, is a testament to the player's ability to kite, avoid taking damage, and stay close to his allies. He must play as a Rifleman. To earn this medal simply survive the campaign, achieve victory, and have less than 10 hits. Hits will only be recorded when the player's shield is breached and Health loss is registered. Having the 20 additional shields from Class Specialization and/or using the XS-4 Armor will improve your chances greatly. On higher difficulties, one of the biggest threats will be the ghouls: they can sprint in Nightmare, and they will crack your shields in only a few hits. When in doubt, run towards your allies. You shouldn't risk jeopardizing the medal over a few meager kills.
As always, the first two Medal Ranks can be earned in Recruit Mode or Normal Mode, but the third and onward are only achieved by playing Nightmare Mode.