[X] "Saber, eliminate Archer. I have some other business to take care of." Assassin, eliminate the target on my mark. (Split your attention between the sniper and Archer.)
[X] Tell Jack to disable the sniper if he tries to take a shot at you. If he doesn't, then tell Jack to follow him and figure out as much as possible about him. Pull back with Saber. Though you may be able to win against Archer, Lancer is still there, and all the other servants and, by extension, masters know you're there. There's too much of a risk of interference.