Poisoning water is off limits ,that's chemical warfare of worst worst kind ,that will result in complete destruction of pakis and international community wouldn't bat a eye.
What's your take on small modified drones capable of carrying 10-20 kg of explosives launched into crowds on a massive scale coordinated during festive seasons say a 1000 of them all over India .
OR small rocket missile type( modified toy) built to carry similar load ,launched from roof tops etc.
Or
multiple blowing of buses ,say somehow plant a bomb on buses in depot and they go off while they are full of passengers ,it will be chaos , multiple buses going off on different locations/routes.
Heck even metro can be targeted , at few places Delhi metro runs parallel to highway ,can easily targeted with a drone or some skilled break and enter guy can even climb on station from there.