I can’t start the mission!

Hello, I'm having some trouble completing the Ranger - Refugees mission in Heroes of Might and Magic V. To recap the mission: you need to defend the borders. I've been trying for several days now and can't seem to pass it; there's a task, but there's no way for the player to survive. I'm deliberately losing with heroes that I have, but I can't lose the mission. PLEASE HELP if anyone knows anything about this. Thank you in advance!