Servants for Kara no Kyoukai ?

KnK event feature a wide range of enemy class and with various difficulty level so there's no single good answer for your question..

However majority of the enemy you will farm midway are Rider and Assassin,, mainly you will need to farm a lot of Rider ghost midway but they're not that hard to do.. Shiki could one shot most of the mob and the SSR one can wipe most enemy in this event except the servant..

There is some servant enemy with pretty wide range of class but nothing unbeatable with good support.. oh there's one quest at the end with EMIYA, MHX and Babbage.. other than that nothing particularly stood out.

The tricky part will be the farming, Since you have saber alter, that's a good thing because you will want her to clear the mob waves. Most of your other servants seem good for now. Though you might want to pack some AoE damage. Don't forget you got a lot of blazes and fires of wisdom you can trade in this event for bottled water. Good luck and have fun