Warlock – “Expansion” [4th campaign, 2nd mission]

I've tried different approaches. First, I gathered troops for two weeks, moved up, but the Rust Dragons kept coming one after another, and they also flee the battlefield, the bastards. I can handle them three or four times, but then they overwhelm me.
I tried using the starting army – it's the same, I don't have time to reach his castle – they keep coming and destroying my army much faster than I can replenish it.
Tell me, do I need to leave troops at the gates, and how do I deal with them in general?