Heart of the Night

Does anyone have anything to say about Gilva? You can send me a private message... I have an idea, but I don't have the energy to implement and test it. I want to find a definitive solution. Thank you.

Look at Remix – place unicorns with magic resistance near archers and other fragile units (hopefully, they won't be poisoned) and shoot until the enemy unit is reduced to zero hit points – the next resurrection will be smaller – and so on until the last unit of that enemy type is gone. It's a hassle, yes, a terrible one.
But most importantly, don't stay inside the castle; instead, stand at its gates (you'll still defend inside the castle) – only then will the script that gives you the green key and some nice artifacts trigger (see my post above).