No, you shouldn't attack Duncan, and it's impossible to do so. If you managed to engage him in battle, it means there's an error in the script. If possible, please share your save file so I can fix it. The task is to assemble an army of 200 archers and purchase a catapult for 50 units of wood and 10,000 gold in the workshop located near the road between Horncrest and Lostdale.
I chose Lorenzo as a secondary hero because his specialization allows for the most effective use of archers, minimizing their losses, as they are crucial for the task.

Thank you for playing and providing feedback. I'm very glad you enjoyed it!