Yes, conventionally, none can defeat India because killing more than one billion people through conventional means is something none can achieve.
There are however two options: one nuclear and that we all know. But there is one more and is more deadly than any nuclear weapon.
India is in a vulnerable geographical position and it has certain natural attributes which made India but also can break India. India is the gift of monsoon which is a very delicate system. With a HAARP like weapon system, one can kill the monsoon or change the way it is working right now.
Now, you may say, monsoon does not matter to you. I give you a clue, the recent flood of Uttarakhand stands as a burning example of what a screwed monsoon can do indiscriminately.
Whoever wants to conquer South Asia and wipe off all the natural habitat (flora and fauna) without firing a bullet, must immediately concentrate on how to kill the monsoon with the use of HAARP like ultimate weapon system.
Who can fight if such clouds (image of Lucknow, UP) start acid rains, or radio active rains, or with high precipitation or no rain at all.
I think more than any nuclear catastrophe, weaponizing natural attributes, particularly the meteorological ones, can result in maximum damages and permanently change the ecology and landscape of the region. For example, if one can kill the monsoon which is a very delicate system and therefore easy to be destabilized, one can change the whole story of this region. Forget humans, even grasses will stop growing.
Just go to the Google Map, and see India. Yes the green patches you see in the images are the tropical rain forests which exist because of the monsoon. Now imagine, the map without the greenery. If that kind of vegetation disappears resulting in the desertification of the landscape, population will overnight reduce from one billion to a few thousands perhaps, if some manage to survive.
If anyone wants to have more discussion on how a HAARP like weapon can change the story of India, please put your queries first.