On one hand the text tells us that Shikhandi in his past birth was a rakshasa
Know that maharatha Dhrishtadyumna was born from Agni's part. O king! Know that the male-female Shikhandi was born from a rakshasa.
- 61, Sambhava Parva, Mahabharata.
With Droupadi, Dhrishtadyumna was born from the fire, as parts of the auspicious fire god. Know that Shikhandi was a rakshasa.
- 1970 (39), Putra Darshana Parva, Mahabharata.
A second version about his previous birth has him being Amba. This is the more popular story
The Mahadeva, with the bull on his banner, spoke truthfully to the maiden. ‘O fortunate one! I do not utter false words. What I have said will come true. You will attain manhood and will kill Bhishma in battle. When you enter another body, you will remember everything. You will be born as a maharatha in Drupada’s lineage. You will be an extremely honoured warrior who is swift in the use of weapons. O fortunate one! Everything will be exactly as I have said it will be. You will become a man after some time has passed.
- 851 (188), Amba Upakhyana parva, Mahabharata.
Bhishma had a vow to never attack Shikhandi
When he appears before me with a bow in his hand, desiring to fight, I will not glance at him even for an instant and will not strike. This has always been my vow and it is renowned throughout the earth. A woman, one who has earlier been a woman, one who has the name of a woman and one who has the form of a woman -I will not shoot arrows at them. O descendant of the Kourava lineage! Because of this reason, I will not kill Shikhandi.
- 856 (193), Ambopakhyana parva, Mahabharata.
Bhishma clearly says he will not shoot arrows at Shikhandi or strike him. However Bhishma himself has broken this vow.
However in the same text, Bhishma attacks Shikhandi multiple times
Shikhandi’s weapon had been sliced down by the king who was Shantanu’s son. On seeing that he was running away in that battle, the great-souled Ajatashatru became wrathful and spoke these words of anger to Shikhandi. ‘You spoke these words to me, in the presence of your father. “Using arrows that are clear and have the complexion of the sun, I will kill Bhishma, who is great in his vows. I say this truthfully.” This was the pledge you took and you are not making it come true. You are not killing Devavrata in battle. O brave one among men! You have become false in your oath. Protect your dharma and the fame of your lineage. Behold. Bhishma is fighting with terrible force. He is tormenting the masses of my soldiers. The net of his arrows is fierce in its energy. Like Death himself, he is killing everything in an instant. Your bow has been sliced down by the king who is Shantanu’s son. You have been vanquished and are running away from the field of battle. You are abandoning your relatives and your brothers.
- 941 (81), Bhishma Vadha parva, Mahabharata.
Drupada pierced Bhishma with twenty-five arrows, Virata with ten arrows and Shikhandi with twenty-five. O great king! The great-souled Bhishma was pierced in that battle and was as beautiful as a blossoming red ashoka tree in the spring. Gangeya pierced them back with three arrows that travelled straight.
- 959 (99), Bhishma Vadha parva, Mahabharata.
The valiant Dhrishtaketu, the king of Panchala, Pandava Bhimasena, Parshata Dhrishtadyumna, the twins, Chekitana, the five from Kekaya, Satyaki, Subhadra’s son, Ghatotkacha, Droupadi’s sons, Shikhandi, the brave Kuntibhoja, Susharma, Virata and many other immensely strong ones among the Pandaveyas were oppressed by Bhishma’s arrows and were immersed in an ocean of grief.
- 973 (113), Bhishma Vadha parva, Mahabharata.
A popular myth about Shikhandi is that Bhishma on 10th day cast aside his weapons on seeing him. But this is simply not true. Arjuna had to forcibly disarm Bhishma
Placing Shikhandi at the forefront, Kiriti impetuously attacked Bhishma and severed his bow. Kiriti, best among rathas, was protected by Krishna and in that battle, after Bhishma’s bow had been severed, pierced him with ten arrows. He struck down his charioteer with ten and his standard with one. Gangeya grasped a bow that was more powerful. However, Phalguna sliced that down with a sharp and broad-headed arrow. Pandava was enraged and one after another, Savyasachi, the scorcher of enemies, severed every bow that Bhishma took up. When the bows were severed, he became wrathful and licked the corners of his mouth great wrath, he grasped a javelin that was capable of shattering mountains. In anger, he hurled this towards Phalguna’s chariot. On seeing it descend, like the flaming vajra, the descendant of the Pandava lineage brought the javelin down with five sharp and broad-headed arrows. O best of the Bharata lineage! When that javelin, hurled angrily by Bhishma’s powerful arms, was severed with five arrows by the enraged Kiriti, it was shattered and fell down on the ground, like lightning dislodged from a mass of clouds. On seeing that the javelin had fallen down, Bhishma was overcome with anger.
- 974 (114), Bhishma Vadha parva, Mahabharata.
So in summary
A. Shikhandi was a rakshasa in previous birth or was Amba.
B. Bhishma apparently vowed to never fire arrows at Shikhandi but did so many times.
C. Shikhandi’s presence did not make Bhishma cast away his weapons and he had to be defeated by Arjuna by force.