Q: What sort of technique was the "Shooting of a Hundred Heads" that Shirou used in Sakura's route?

A: The Noble Phantasm of Heroic Spirit Heracles is an all-purpose Noble Phantasm that he changes according to the target and situation. Bluntly put, it's the "Shooting of a Hundred Heads Style."
What Shirou used was the anti-personnel version of Shooting of a Hundred Heads. Honestly, just think of it as nine consecutive high-speed attacks. By the way, the Shooting of a Hundred Heads that Heracles mainly uses is the anti-Phantasmal Species version, which is like nine dragon-type homing lasers fired simultaneously. What the hell, man?

Comptiq 2007 Issue 3; Fate Dojo Q&A