Frodo, Sam, Boromir, Boromir!lives AU, gen, Amon Hen to Amon Amarth

On the Stairs of Cirith Ungol, after they find Frodo cut loose from Shelob's webbing and signs of her having been stabbed, Gorbag accuses Shagrat "‘By all the signs, Captain Shagrat, I’d say there’s a large warrior loose, Elf most likely, with an elf-sword anyway, and an axe as well maybe; and he’s loose in your bounds, too, and you’ve never spotted him.
Boromir is that warrior.