I've read this fic several months back and I've been trying to find it again. Google turns out too many (and more often than not, I end up with the episode summary of 'The Gift').
It is set sometime during that season but they found a way to save Dawn. Buffy and Giles went with Dawn to hide in some other realm while Xander, Spike, Willow, Tara (and Anya, I think) bought time. They just have to hold Glory off until the next dawn/sunrise( or was it sunset...) since the portal can only be opened on that specific day and no later.
The distinct part here that I'll always remember is Xander and Spike were still not exactly on friendly terms but they had to work together. Xander used up his soldier memory, brought a sniper along and shot the medieval knights on their limbs, trying not to kill them but immobilize them. When they grew desperate, Xander "let loose" Spike and allowed the killing and even Xander was taking fatal shots on the humans. And I think it was Willow who was shocked that first, Spike was listening to Xander (in reining in Spike's violence) and second, when Xander released Spike and he went for the kill.
I remember Xander shooting a guy on the head who was about to kill Tara.
There was also a church and nuns and they took refuge there while Xander stood on some high ground sniping. Willow casting fire spells. Spike...killing.
Ok that was long. I hope someone's read this and can point me to it.
FIC FOUND: Career Change. Thanks reremouse! That was fast lol I was about to add another detail remembered but then the fic's been found. <33