Nevertheless, I'll have to replay the 3rd campaign, although I was about a month or two away from finishing it.
I made the wrong choices in character development. I left luck as a stat, but I should have focused on mentoring, because without it, the Dark Elf won't gain enough experience in the 5th campaign. The mentor's guard in the 5th campaign isn't very durable, and there are a lot of Vampire Princes, but my Orc can travel between cities using portals, so I can feed the Elf experience that way. The only problem is that the Orcish capital is under threat, so I need to protect it. And I need to send the mentor out from the third mission.