Simply impossible ? No , it isn't . Pakistan has an ever expanding nuclear arsenal currently at 120 warheads and diverse delivery systems . The intention is to cause unacceptable damage from where there is no going back - a point of no return if you will . The complete destruction or annihilation you talk about is an outdated concept .
Pakistan's access to fissile material is limited - unlike India where NSG waiver has opened up option of our domestic uranium to be completely diverted to weapons production. FBR and 3 stage nuclear programme will increase fissile material production 10 fold that of Pakistan .
Pakistan does not have long range missiles to cover whole of India. Both quality and quantity of such missiles are limited .
Pakistan has no thermo-nuclear weapons ....
Pakistan's first strike capability is also therefore severely limited .
India will never be first to use nukes .and If Pakistan uses nukes first - then India will have sympathy of international community.
Pakistan has to therefore desist temptation of using nukes against india .