Alkeides judged that it was Berserker’s Master and immediately grasped what the shadows meant.
They were merely a diversion.
If they had been a type of magecraft that inflicted direct harm, he would have been impervious to them.
It would be a different story if his adversary were a mage from the Age of Gods, but assuming that Berserker’s Master was a human mage and not a Heroic Spirit, that was impossible.
According to intelligence supplied by his own Master, Bazdilot Cordelion, Berserker’s Master was a genius from the headquarters of the Mages Association known as the Clock Tower. As long as he was a modern mage, however, his magecraft was nothing to fear, and the mage ought to be equally aware of that.
In which case, he ought to regard the shadows as a diversion.

Fate/Strange Fake: Volume 5, Chapter 15 ("Gold & Lions II")