Xel
The Paladin
I think Rider and co mentioned a possible opening also, She can retain manifestation for aboooout a week if she not hurt, combine that with her hiding skill and she could probably ditch a master wait out the bloodiest of the war, grab the master most likely to win and make off like a banit. Besides, it wouldn't be a fate series if at least one of the archers wasn't a rebellious self focused fighter.
Well, I'm specifically talking about her still being combat capable, not merely surviving. Team Saber's probably the only one that could have Archer and actually use her. Maybe Team Caster because Caster's are cheating dicks. Anyone else and Archer would basically be nothing more than decoration because all her mana has to go to her own upkeep.