To kill Quill the sniper very easily, when you start the level, run into the adjoining bunker on the other side. Then, blow a hole through the wall. After that, blow up the pillar in front of you as much as you can until you see her. Take out your sniper rifle and keep shooting her in the head. This is the easy way to kill her under any difficulty setting. If you run out into the room, there are guards with rail guns, sniper rifles, and nano civilians. By staying in the bunker, you only have to worry about shooting Quill, who hardly hits you. You will only have to worry about a few nano civilians. The snipers and rail gunners can be ignored.