It was Arjun who cut off Bheeshma's weapons and Bheeshma surrendered to Arjuna, the battle in description is all about Arjuna fighting Bheeshma and Shikhandi hitting him with arrows here and there, Those arrows didn't even stop Bheeshma from fighting, Bheeshma was coming down from his chariot with a sword to fight Arjun but Arjun cut off his sword too
Shikhandi was there so were other warriors with their arrows and maces who rushed towards Bheeshma after Arjun cut his sword, It is mentioned that Kuru army afraid of Arjuna abandoned Bheeshma
"Having slain in that battle, O monarch, (his foes) by hundreds and thousands, there was not in Bhishma's body space of even two fingers' breadth that was not pierced with arrows. Thus was thy sire mangled with arrows of keen points by Phalguni in that battle. And then he fell down from his car with his head to the east, a little before sunset"
There's long passage where Shikhandi is not mentioned during the war and during Bheeshma dies
Shikhandi killing Bheeshma is mentioned twice and both times it is attached with divinity and reincarnation of Amba
Arjun was a very willing participant after Krushna emotionally blackmailed him, Krushna was about to use his weapon on day 9 against Bheeshma breaking his vow which would have resulted in Krushna killing himself, Arjun stopped him and next day he participated with full force, You can read him laughing at Bheeshma, full of force, you know... adrenaline rush..