Who killed bhishma of mahabharta?

Answer 1

Arjuna killed Bhishma in the war of Kurukshetra, by using Shikandi as a shield. Knowing that Shikandi was originally a woman, Bhishma would not attack her, and thus got killed.

Shikandi was Amba in his/her previous birth. After obtaining the boon that she would kill Bhishma in her next birth, Amba committed suicide to hasten that birth. (Read that story here). She was born as a daughter of Drupada. While still a child, she wore a garland of flowers, which had the special property that the wearer would be able to kill Bhishma. Fearing the great warrior's wrath, Drupada exiled his daughter to the forest. There the girl met a Gandharva, who offered to exchange her female form for his male. By then the girl knew of her destiny, and gladly accepted. When he(she) came back, she enrolled as a warrior in the army of Drupada and rose to the rank of a commander