Saber’s magic resistance ability can only be activated when someone targets her to use magecraft. It can not stop Caster from summoning monsters from other worlds. Moreover, once those monsters are summoned, they would materialize and carry a threat different from magecraft. The teeth and curved claws of the monsters have attack powers equal to swords. The only things Saber could use are her sword and body when facing those monsters.

- Fate/Zero: Act 6, Part 2