For those of you having trouble with "Fires of Calcutta" on Hard difficulty, Mahouts are the best unit for the mission

If you can get out even a few Mahouts, they will easily decimate the enemies that come at you. The Arsonists and Chakrams that come at you count as Siege and Ranged Infantry units, respectively, which Mahouts excel against.

Early on, once you reach the town center, try to use shipment cards to get some Sowars and Zamburaks at first, as you can't easily get Mahouts via shipments. The enemy unit spam is strong against infantry, so try not to make more than what you start with; cavalry will be much better, even if it's very expensive. One big misconception about the mission is that some people think Gurkhas will do well against the Arsonists, but that isn't the case. Arsonists have 50% ranged resist, even if they count as Heavy Infantry, so Gurkhas don't hurt them as much as you'd think.

Building Castles will slow the enemy down a bit, but they won't stop them, as the enemies do plenty of siege damage. Still, it takes a while to have a strong enough economy to support Mahouts, so do whatever you can to slow the enemy down while you build up.

As for your economy, don't try to send your villagers too far out or they'll get sniped. This unfortunately will leave you with a shortage of resources, so try to get Rice Paddies up if you can. Wood is the main issue, as it always is with Indians; you'll want at least one infinite Wood shipment.

This mission is definitely a painintheass, don't get me wrong. You can't make walls, destroying the barracks won't stop enemies from spawning, and you start in the town with zero resources (and can't afford to go for treasures early). It's definitely something the AoE team should've looked at adjusting, IMO, even if it's doable with the right strategy. But once you have your Mahouts out, you can generally just stomp them toward the barracks and flatten them.