[quote=@Flamelord]
<Snipped quote by Sublimation>

Depends on the Servant really. Rin spent up almost ten years worth of stored Prana to kill Herakles once, which isn't exactly a feat you can do more than once on short notice. 

I would argue that Kuzuki would have lost had that fight gone on much longer, since Saber was becoming accustomed to his fighting style. Which is why he bugged out when he did. And I seem to recall most of Emiya's fight with Lancer involving him running away and getting his ass kicked, with nary a chance of him actually winning in sight. 
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This is Heracles we are talking about, who deflected three A ranked ice spears that Rin made and was only killed when his A+ Strength grip was countered and he was surprised by FIVE A ranked blasts of light. Rin wanted to go overkill, as was specifically stated in the VN. She originally believed only 2 or 3 jewels was enough to beat Heracles, but Shirou and Saber forced her to use at least 5 in order to be safe.

One A ranked attack can easily destroy a servant if it hits directly. All of Medea's beams would have disintegrated Archer instantly. Kuzuki would not have lost that fight had he not killed Saber the first time he skirmished her. His Snake style is bred from a mountain assassin school in Fuyuki where literally all you do is train in assassination to kill the enemy in as short a time as possible. Its the ultimate assassination technique that focuses entirely on strangeness to confuse an opponent. Kuzuki himself does not match to servants, but his fighting style does. Bazett can defeat servants quite handily too.
Not many masters have been top grade in terms of modern potential anyway, with people like Uryuu and Waver and blah. It is extremely hard to defeat a servant of course, but its not as hard as you think it is. You just need to know some of the more obscure stuff around in the Nasuverse.