Winchesters if you are going by the TV shows. Buffy gets tagged a lot. I see no reason she would be able to dodge two sharpshooters. She was reduced to being pinned down behind a bar once Darla started shooting dual pistols (with little seeming skill) in "Angel" is the seventh episode of the first season of Buffy the Vampire Slayer. Not sure what she would do versus a shotgun.
First season Buffy does not equate to final season Buffy, who was dodging automatic gunfire and dealing with armed soldiers without issue in later seasons. She increased in reflexes, speed, physical stats and skill as the show went on. As for the shotgun, they don't have that listed in their equipment in this battle.