The most challenging campaign.

Spent 1 week settling into the Inferno castle (market, warehouse, drilling for imps and demons). Mid-week 2 – we defeat Gilraen and head straight to the nearest Elven castle (Vinlen, I think) – the main goal is to capture it before the end of week 2.

This is a huge mistake. Capturing this castle triggers a script that starts spawning Elven troops. Do we need that?

In reality, everything is much simpler.
Week 1 – we defend the mines and gather resources – our goal by the end of the week is to build Cerberuses. At the beginning of week 2, we upgrade them in a chain and transfer them to the main hero.
Week 2 – we use Cerberuses to defeat the Earth Elementals (it’s very good if there was a hero with dog units in the tavern), which block the path to the 2nd (upper) Elven castle. At the end of week 2, we destroy this castle. At the beginning of week 3, we buy reinforcements and quickly capture the mines, moving towards the castle where Gilraen is located. We build a chain from the Elven castle. We stop at a distance of 1 full turn from the castle. Gilraen has no logistics, but we do, which means he is not a threat to us – he won’t be able to reach us. We wait for him to leave the castle and immediately capture it (it’s empty). At the same time, we capture the castle that was closest to us. Gilraen attacks (he has no cities) – we defeat him – the mission is completed. It takes less than a month.
If he was afraid to attack (unlikely, but still) and ran towards an empty castle, we leave the castle. He runs back. We occupy it again. And so on for a week. In the end, he dies, and the mission is completed.
I played this on Hero difficulty.