Well crap, guess I picked the wrong choice. Honestly I should have expected saber to overwhelm them so quickly that they couldn't get to the castle in time, having seen Heavens Feel, but at the same time I kind of thought that the witches could fly faster. Or at least above the trees.
[X] [Summon Mordred]
At this point, I can't see any other viable option. Direct combat at this point with what we have is just flat out suicide, with salter being practicality impervious to magic and being strong enough to overpower Herakles. She can close distance too quickly to retreat, blast away all cover, and even immobilize us through sheer presence alone. And no matter how much we might want to reach Rin, to talk or beg or kill her, we still run into the same problem where Salter will kill us in the next 30 seconds, probably long before we can even locate Rin. Hell, it's not even guaranteed that she's nearby. It's not like Kirei was tailing Lancer in canon whenever he sent him to scout, he was probably miles away.
Mordred is a new element we can bring in to change the odds, someone who is built for close combat, especially for Artoria specifically. And it probably won't be to hard to point her in Salter's direction.