Discrediting panchnama and panchas (witnesses)

In a criminal case related to engineering works spanning over 56 work sites, observation memos or panchnamas were created by the investigating officer and 3 to 4 panchas or witnesses on each site. Only the IO & one witness were common on each site. Certain items were in possession of the different witnesses since earlier dates than on which the investigation was conducted. But on the panchnama, all the panchas including the IO have signed and marked those items to be found missing. Later on, the IO obtained letter from the department having clear mention of the items being handed over to some of the witnesses. While filing the chargesheet in the court, those facts were not disclosed and the items in possession of the witnesses haven't been removed and blame has been imposed on accused. Trial is in process in special court. Does the accused have to discredit the credibility of the panchnamas and the witness of each site one by one OR by discrediting one or two panchnamas, credibility of the whole chargesheet and the common witnesses is diminished at a time?