And boosting defense for an Orc isn't the best option.

I already wrote that you should boost defense for an Alt hero. The chance of an Alt hero getting it is significant. Shakh'Karakat, with a bit of luck, can get "Protect Us All" at level 3. At worst, it's level 4-6. Also, you can level up an Alt not only at Dolmens of Knowledge and Magic Temples, but you can give all the Orcish units to Shakh'Karakat (or another hero who got the defense skill), and after killing the mobs guarding the passage to a neutral castle, send him there to capture a mine, sawmill, and maybe even the castle. This accomplishes two things at once: you get a bunch of goblins and, as a bonus, you get stones and wood, which are often lacking when upgrading your buildings. At the same time, your main hero doesn't have to travel far and leave your home castle unguarded.