Instructions
Go into Entrance Plaza at 4 pm on Sept. 20 or within about 24 hours of that time. A short scene will play out showing you a family of snipers on the second level that have a survivor pinned down in a shop. Go up the stairs and back toward the snipers.
Enter the sports shop close to the snipers. You should also have a strong-bladed melee weapon equipped such as a katana, hunting knife or machete. Wait for one of the snipers to run by the store and chase them out.
Swing your melee weapon as you're chasing the first sniper. He will run away from you while crouching down, so you'll probably only be able to get one or two shots in at a time. Keep chasing him and swinging your bladed weapon until the first sniper is defeated.
Go back into the sports store and wait for another sniper to go by the entrance. Chase this sniper and kill them using the same method as you did for the first one. Once you're down to one sniper you can charge at him. He will be shooting at you while you rush him, but the sniper rifle is slow to load and you can avoid his shots by approaching him in a zig-zag motion.
Chase the last sniper down while swinging your bladed weapon at him. Keep attacking until the last sniper is dead.