Dronanca Bhishmanca Jayadrathanca|
Karnam Tathaanyaanapi Yodhaveeraan||
Mayaa Hantaanstvam Jahi Maa Vyatishtitaa|
Yudhyasva Jetaaasi Saptanaan||
Slay Drona, Bhishma, Jayadratha, Karna, and other great warriors who are already killed by Me|
Do not be distressed, fight, and you will surely conquer your enemies in battle.
This verse simply means that Krishna"God himself"had recognized that this was to be Bhishma's ultimate for siding with Adharma (injustice, non-righteousness, etc) and that Arjuna's duty was to merely put an end to Bhishma's physical body.
(Thanks a lot for sharing this verse from Bhagavad Gita Srujadiđ¤)