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The mission is actually very easy. Here's what you need to do: start with two heroes. Divide the armies so that Irina's army is weaker (but her magic is strong), and use the second hero to capture the neutral Castle and everything around it, while Irina explores the sea in search of resources, etc. After capturing the Castle and gathering resources, direct both heroes to the gladiatorial arena, and then to the second Castle. Give Irina the entire army for one battle and capture the Castle. Now, split into two fronts. The second hero goes to the gate near the neutral Castle, while Irina goes to the second city. Capture everything there (and everything along the way), and hire a third hero. For now, only Irina carries the army. Irina captures the second city but doesn't seek help from Kirin; instead, she goes for Ronin Tiangbo. Hire a fourth hero and send him to seek help from Kirin. Meanwhile, build a powerful army with the third hero and transfer it to the second hero, who is near the neutral Castle. Now that you have Kirin, Tiangbo, and the second Castle, sell the fourth hero (and replace him with Tiangbo). Capture the city below the second city (Shondze, I think), and with the second hero, go south from the first city. Capture the city before Shondze (or something like that). Now, all the heroes go to the second hero, where you transfer the entire army to Irina. (I had an army like this: 499 Vanadzame, 603 Pearl Maidens, 400 Kap-Soi, 175 Spring Spirits, 100 Snow Spirits, 200 Kenshi, and 56 Kirin. Capturing Shondze won't be difficult.) Then, the gates to Hiroshi open, and he surrenders to you; the mission is complete!
Added 2 minutes ago
I completed it in 3 months