Limited nuclear war with Pakistan would be the obvious choice. Another thing you could try and do is avert/severely mismanage the Green Revolution. India used to be heavily dependent on food imports and many were predicting that it was heading towards mass starvation in the 60s, but after the Green Revolution it became a major food exporter (which isn't to say that the Green Revolution was all sunshine and rainbows). There were also some nasty droughts in the 60s and 70s which IOTL the Indian government was able to deal with effectively. Have them massively screw the pooch on those and you could easily rack up a pretty high death toll.
It worries me that I've put so much thought into devastating India.