Use Geo Zones. Set up the areas you want in System > Localisation > Geo Zones, then in the shipping method settings (Extensions > Shipping) you can choose where that shipping method is available. Choose your "India" geo zone for Free Shipping, and your "Worldwide" geo zone for Flat Rate.
OpenCart forces you to manually choose every country in a geo zone, so if you don't want to manually set up your worldwide geo zone, then
Total-Based Shipping can do this. For example:
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CHARGE #1
Title: Free Shipping
Sort Order: 1
Charge Field: 0
Rule: Geo Zone is India
CHARGE #2
Title: Flat Rate
Sort Order: 2
Charge Field: 2000
Rule: Geo Zone is not India
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Feel free to take a look at the screenshots and demo site, and if you're interested let me know at
www.getclearthinking.com/contact if you have any further questions.