It was Bhishmadeva's duty to conduct the fight (the Battle of Kuruksetra), and no pains would be spared in that connection. BG 1972 purports

Expressions researched:
"still, it was his duty to conduct the fight, and no pains would be spared in that connection"

BG Chapters 1 - 6

Indirectly, by the symbolism of the conchshell, he informed his depressed grandson Duryodhana that he had no chance of victory in the battle, because the Supreme Lord Kṛṣṇa was on the other side. But still, it was his duty to conduct the fight, and no pains would be spared in that connection.

BG 1.12, Translation and Purport:

Then Bhīṣma, the great valiant grandsire of the Kuru dynasty, the grandfather of the fighters, blew his conchshell very loudly, making a sound like the roar of a lion, giving Duryodhana joy.

The grandsire of the Kuru dynasty could understand the inner meaning of the heart of his grandson Duryodhana, and out of his natural compassion for him he tried to cheer him by blowing his conchshell very loudly, befitting his position as a lion. Indirectly, by the symbolism of the conchshell, he informed his depressed grandson Duryodhana that he had no chance of victory in the battle, because the Supreme Lord Kṛṣṇa was on the other side. But still, it was his duty to conduct the fight, and no pains would be spared in that connection.