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The second mission caused me some serious difficulties (the first was the last mission for Sanctuary).
So, here's my tactic:
Two starting bonuses (experience and a free hero). At the beginning, I leave Sandor in the Castle, buy two more heroes (one for free, as a bonus); I fully upgrade their Economy and Logistics skills (the rest is optional; I upgraded Luck, Diplomacy, and Experience). I send Airini to the top to the first available veteran units, two Orcs down, one to the lower left corner, and the other to the Goblins. Then, Airini goes to the nearest Dungeon (where there are veteran Panthers). It takes less than a week, then we send all three Orcs to gather units and resources near them (in the city, we build a teleport and upgrade the Orcs; if necessary, Sandor teleports and repels the attack). Then, we gather all three Orcs in one place and transfer all the armies to Airini. She, with the second Orc (it's better to take the one with better movement), goes to the remaining veteran units on the left side (Cyclops and someone else), while freeing the artifact shop from neutral creatures. We leave our accompanying Orc there (he will buy the necessary artifacts). The third Orc goes to the sea to gather resources. After collecting all the units, we teleport Airini to the Castle and switch places with Sandor (including the armies; that is, Sandor goes with the un-upgraded units, and Airini stays with the upgraded ones – by this time, all the upgrades in the Castle should be built). The third Orc, after clearing the sea, returns to the Castle; the second continues to buy artifacts. Sandor goes to the Dungeon, where he meets the Necromancers, who give him the horn. Then he goes to the Harpies, visits them, and collects all the nearby neutral Orc units. After that, he goes along the road leading to the Dragon, where he makes a choice: give the horn or fight the Dragon (tears or blood; I chose the path of tears, so I gave the horn). After that, Sandor teleports to the Castle, upgrades the units, and waits for guests. Ideally, by this time, you should have good artifacts (for example, the Emerald Bow of Yumi). With such an army, any enemy will seem easy, and you will easily level up Sandor to the required level. You can run around and clear the map to level up Airini as well, but on the other hand, there is the final campaign and the 4th mission, which is very profitable in terms of experience; it's up to you