Lina Shields said:
- Boros will realize it immediately due to the fact that Excalibur needs significant time to charge.
First, Boros has no knowledge whatsoever on what Excalibur does, he has no reason to stop his charge
especially since he's bloodlusted. His effective range is h2h only and starting distance is 4 kilometers, you do the math.
Also Excalibur doesn't take time to charge, like at all. I REALLY should put a Note on her profile. If you're referring to Fate/Zero Episode 15 with all of Irisviel's monologue that doesn't exist in the novel. Saber just raises the shining sword and brings it down. Iskandar reached the bridge where Gilgamesh was standing and watching just as Excalibur's light was unleashed. The Bridge was no more than 100-200m away if not less.
Also Saber activated it "instantaneously" in her fight against Iskandar
Fate/Zero, Vol.4, Act 14, Page 95
The King of Conquerors' direct attack sent a shiver down Saber's spine.
The rushing of the divine bulls had covered the hundred- meter distance instantaneously. In the blink of an eye, the might of Gordius Wheel manifested before her eyes. If the hilt of her precious sword had still been in her hand, she would have been certain of victory; facing the golden radiance raised by Rider, there was only one true name she could shout. "Ex—" As the rushing incarnation of thunder was about to trample Saber's small frame ... "—Calibur!" Golden lightning, with the radiance of countless comets, lit up the night like day.
And yes Excalibur attacks every single atom of the surface area it touches.
Fate/Zero, Vol.3, Act 10, Pages 98/99
"Excalibur!"
The light roared. Prana, accelerated by the released dragon factor, became a streak of light, a swirling and surging torrent that devoured the sea demon together with the dark night.
A silent scream rose from the evaporating river water as every single atom of the giant sea demon—once the embodiment of terror—was exposed to the scorching impact.
If Saber can eradicate a humongous monster hundreds of times bigger, Boros is not a problem. Especially since he has no knowledge on her sword, what it can do, he's bloodlusted and doesn't care about damage (also he would be confident in his regen) and starts from a 4 km distance.