So, to answer the famous question, yes, Shiki is absolutely capable of killing Servants if she could contend with and kill Araya, since average Dead Apostle Ancestors can keep up with Servants and Araya is at least a rough equal of Nrvnqsr, who is an elite among the elite according to Arcueid, putting him far above normal DAAs.
To elaborate further : Servants are compared to most Dead Apostle Ancestors. The thing is though, every Dead Apostle Ancestor we've seen has always been some of the strongest ones that are far above the others. Nrvnqsr states the combination of his lives requires the power to obliterate a continent to fully destroy, with speed stated to match Arcueid's when he goes all out. Nrvnqsr is the elite among the elite regarding the Dead Apostle Ancestors. We've actually never seen an average Dead Apostle Ancestor in the original Tsukihime canon ever. Servants most certainly are not taking him on. The speed difference alone also makes up for any potential hax they may have. They mostly have subsonic to supersonic feats and scale to Beowulf from Mahoutsukai no Yoru, who has subsonic speed and beat a basic Dead Apostle Ancestor, who is a general equal of Servants. Meanwhile most Tsukihime characters scale to Roa's lightning+ speed, as they can avoid getting blitzed by him, which puts them around potentially a hundred times faster than Servants. in Actress Again, Roa appears in a Tatari manifestation and is capable of using his magecraft again, which are lightning attacks. And since magecraft is basically the creation of natural phenomena through supernatural means, it should be regarded as genuine lightning, especially since he uses it to amp his speed. I have no idea why he would use it to amp his speed if it wasn't akin to real lightning. And since Roa in this manifestation isn't really that impressive compared to everyone else, his speed acts more as a baseline speed for Tsukihime, since he never blitzes anyone. And they aren't aim-dodging his attacks since again, he uses the lightning to amp his own speed for more than just one attack.