Magecraft can harm servants if it powerful enough even from a Master.
heres the info on it form the wiki
Magic Resistance (対魔力, Tai Maryoku?, localized as Anti-Magic) grants protection against magical effects. Differently from the Resistance effect that merely rejects prana, this ability cancels the spells altogether.[1]
EX: In addition to the Magic Resistance of the Saber Class, Joan demonstrates high Magic Resistance due to her unwavering piety. However, since it is just averting (evading) the Magecraft, only Joan will be saved against a wide-range magic attack. It cannot cope with the sacraments of the Church either.
A: Cancel spells of A-Rank or below. In practice, the Servant is untouchable to modern magi, so it would not be an exaggeration to title the Servant a "Magus Killer".
A: In the case of Elizabeth Bathory, this ability exists as a hidden skill that the Servant herself isn't really aware of -- active due to the actual presence of draconian blood in her ancestry, rather than as a phenomenon generated by Innocent Monster. Renders a Magic Resistance comparable to that of the Saber of Fate/Stay Night.
B: Cancel spells with a chant below three verses. Even if targeted by High-Thaumaturgy and Greater Rituals, it is difficult for him to be affected. (As Saber Alter, Arturia suffered a rank-down in this skill due her corruption)
C: Cancel spells with a chant below two verses. Cannot defend against Magecraft on the level of High-Thaumaturgy and Greater Rituals. (Because she herself does not have any Magic Resistance, Nero boasts a low level that is unbecoming of the Saber Class)
D: Cancel Single-Action spells. Magic Resistance of the same degree of an amulet that rejects magical energy.
E: Cannot cancel spells, but magic damage is reduced somewhat. (Lancelot's originates from a magical ring, however its powers received a rank-down due to Mad Enhancement)