"Escape to Salvation" - "Magnificent Retreat" [Campaign 4, Mission 4]

I have a problem in this mission: as soon as I capture the third mine, a dwarf appears and attacks the demon city, ultimately losing. I can't seem to fix this. Can you help?

As they say, help yourself! :) The key to success in this mission lies in properly developing your heroes in the previous missions. Retrain Wolfshtein (for example, with swiftness, ballista, runic weapons, and righteous fury – the rest is up to you), and you'll solve one problem.