Sanctuary – “Life in death and death in life” [Campaign 4 – Mission 4]

Anything is possible. For example, I didn't buy an army for Irina at all until I captured two more towns. That is, I killed everyone in the area with the starting army that she has. Elementals are great. I summoned a water elemental, gave it regeneration, and sent it into battle – that's the whole process of the initial playthrough. I played on normal difficulty.