Sorry for click bait, but yeah. Anywhoo. So my fireteam went in with the hope that as long as each of our last shots on both brothers were with a shotgun we would all get credit, no matter who killed them. So that checks off. One kill gives the whole fireteam credit. But it gets better. I never hit one of the shield brothers with a shotgun before he died, I was busy clearing ads, and I still got credit for it. Only the killing blow needs to be with a shotgun, and it doesn't matter who does it. TL;DR - Only one person in the fireteam needs to land a killing blow to the shield bros with a shotgun. The other two don't even need to touch them.