<Snipped quote by Flamelord>
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.
This....actually proves my point. Besides Rin, who spent years storing up Prana to do this sort of thing...
Emiya = Hax magic in the form of near perfect replication of Noble Phantasms
Kuzuki = Special ability as a result from spending his life training in super secret assassin school. Also, getting buffed to like twelves times standard human by Medea
Bazett = Has Fragarach, which reverses the flow of time so it always kills the enemy before their most powerful strike. Though still a skilled mage otherwise.
Kirei = Incredible talented in a lot of areas, though not to the point of mastery, and has hax magic in the form of Command Seals
So, as I said. Generally, the Masters that have the best chance against Servants have some sort of overpowered magic, or special ability.