Walkthrough for the "Tribes of the East" expansion.

Well, if anyone finds it helpful, here's how I leveled up Arantir with Ornella.
These screenshots were taken in the last Necromancer mission, but you can level up the heroes to unlock special abilities as early as the 3rd campaign. The key is to proceed in order, using the hero's skill wheel (the file is located in FanDocuments in the game folder and is called - Skillwheel.exe).