Firstly, lets get this straight. A servant a more powerful than a normal human primarily due to 2 factors: 1. They are spirits, and normal humans cannot hit them at all. So its one sided. 2 They have NPs that goes BOOM! or something to that effect, and you die.
But lets assume you can hit a servant, and that servant is kind enough to let you hit him, a stab through the heart still kills him. Blowing his brains out still kills him. Fine, they may survive a little longer than a normal human, but they still die, most of the time anyway(berserker does not exactly count cos of his NP).
So what can hurt a servant? Servants and other spiritual entities can of course. Magic can too (or caster will be wtf?! useless). But after this, things start to get a little fuzzy.
1. Traced weapons can hurt servants. Fine, you can call it magic, so not much dispute here.
2. Reinforced fists/steel poster can hurt a servant. Kuzuki owned Saber cos of his reinforced body. Shirou block lancer a couple of times due to his reinforced steel poster. So, reinforced things count as magic? And they can harm servants?
So why don't we summon caster, reinforce a sniper riffle, say Barrette, and snipe enemy servants from
00m + away. The bullet will probably get there in about 1 sec or less (reinforced remember?). Bam! 1 down, 5 to go. Especially Caster is living at the temple, with only one way up the mountain. Its practically begging for a sniper.
Don't like snipers? Its ok. Since Caster can detect other servants from afar, she can reinforce a cannon or something, and when a few servants engage, fire away?