Experience was a valuable resource and often a useful weapon, it allowed one to prevent past mistakes and avoid bigger ones, but to get something first it was always necessary to pay a price for it. In the case of experience, the price was always the same: failure.
Adam knew this well, experience allowed him to react lightning fast to the Sacrificial Holder's attack, ignoring the piercing pain in his shoulder.
For the same reason, he didn't stop at a single attack. Most inexperienced Phantoms would not have cut off their opponent's head and continued to attack senselessly, there was no point in being too bloodthirsty and wasting energy.
However, Adam felt that he had to destroy the Sacrificial Holder, he didn't know if the vines could act if the Sacrificial Holder lost its head or not.
He continued to slice with his needle until he turned the monster into meat cubes scattered in front of the gates.
Twitch. Twitch. Twitch.