New viewer, why doesn't Archer headshot Illyasviel ?

I just started Fate/stay night: Unlimited Blade Works and Fate/Stay night in general (followed the recommended watching order on Reddit), and I'm linking it so far (4 eps)

However I'm a bit confused about how fights works: in EP3, they explain that killing the master is the most efficient method to win the war. But a few minutes later, when a fight arise, they decide to Not Do That and focus on Berserker, which is the equivalent of hitting a wall with your fist.

Why doesn't Archer just shoot on Illyasviel ? Surely she isn't as resistant as her servant. Is there an untold rule that prevent them to do that ? Maybe that's spoilers and I should just wait and see ? It's been bothering me for an hour now and I can't get it out of my mind :p