Bhishma was never on par with Drona in knowledge of weapons,arrangement of vyuhas and other related things.

On top of it,Drona was a great instructor who trained the youth of Aryavarth into competent warriors.

Bhishma had the knowledge of Chakravyuha formation.The vyuhas which he formed on ten days of his leadership were not complicated in structure.

Bhishma could handle Arjuna as long as he fights mildly. Those were the instances when Krishna threatened to break his vow and kill Bhishma if Arjuna won’t fight with his full skills.

Once Arjuna unleashes his prowess,Bhishma has no option but to retreat along with his army. Just like it happened when Arjuna hurled Indrastra..Bhishma has no answer for that..

Just like on tenth day,Arjuna successfully guided Sikhandi to Bhishma,steering off all the prime warriors of Kaurava side.So even if Bhishma arranges Chakravyuha with the help of Drona,he can not stop Arjuna from breaking it.

Just like on tenth day,Arjuna successfully guided Sikhandi to Bhishma,steering off all the prime warriors of Kaurava side.So even if Bhishma arranges Chakravyuha with the help of Drona,he can not stop Arjuna from breaking it.

However Bhishma in his ten days of war did not have any strategy to deal with war. He just killed ordinary soldiers..in huge numbers.

The real war of Kurukshetra happened only under the leader ship of Drona….not Bhishma.

Chakra vyuha,Suchi Sakata Chakra vyuha ..all these were Drona’s formations which made celestials wonder and alpplaud.

Seeing that array formed by Drona, which resembled the ocean itself in agitation, the Siddhas and the Charanas were filled with wonder. And all creatures thought that array would devour the whole earth with her mountains and seas and forests, and abounding with diverse things. And king Duryodhana, beholding that mighty array in the form of a Sakata, teeming with carts and men and steeds and elephants, roaring dreadful of wonderful form, and capable of riving the hearts of foes, began to rejoice.'"