Chapter 2,Verse 23

nainam chindanti shastraani
nainam dahati paavakaha
na chainam kledayantyaapaha
na shoshyati maarutaha

Now,Krishna come back to referring to the atma as the eternal spirit.
He says that no weapon(arrow,spear,mace) can cut the atma.
Fire can only burn the body,not the atma.
Water can erode stones over a period of time gradually,but it cannot do the same to the atma.
Wind can never dry the atma.

He points out that none of the pancha buthas(Except for space,as space is compared to the atma in other parts of the Gita) can have any effect on the Atma.

Thus,he encourages Arjuna to fight the battle,as by killing Bheeshma & Drona,he is destroying only their bodies and their atmas would find new ones to inhabit.