Perpetrator: Water of Life
Number: 1,000,000 Hindus converted to Christianity
As a methodology, the Christian organization called Water of Life goes to remote villages in India, where they drill wells, and baptize entire villages who take water from those wells. They run fund raisers to raise funds from innocuous donors for these projects.
They do some infrastructure development and use that as a carrot to mass convert Hindus to Christianity. They are active in many parts of India, with a major ongoing project in Andha Pradesh.