On the really overpowered list of things... Get Sphener. Convert Govanon with Command IV Sphener. Vampires with fireballs.
hah, good luck with that. I don't see many of the AI's heroes lasting very long. If the amurites survive long enough to build govannon, he's not likely to survive long enough for you to capture, before they send him off to die alone.
The calabim don't have any problem fighting off champions, seriously. first of all, the vampire is their champion. and although they have only 5 str instead of 6, they come a fair bit earlier, only requiring feudalism.
In addition, the 1 less strength quickly becomes irrelevant, because you're going to severely outclass those champions in levels, thanks to feeding on cities. Where the enemy has a Combat III champion, you have a Combat V, Shock II, Mobility vampire.
In my current calabim game (emperor difficulty) I've just slaughtered a mass clan invasion, mostly consisting of rangers and champions, by using my ridiculously strong vampires. Valin and Losha helped a bit too, of course
One of the things people often overlook, is that vampires can be quite hard to kill. Every single one of them has Summon Skeleton, and since the vamps have Channeling II, the skeletons will almost always defend the stack over vampires. And since blitz is an extremely rare thing to see, generally a skeleton will die and use up one of the enemy stack's attacks. Not to mention injuring them too if you're in a decent defensive position. You can then just summon a replacement next turn.
It's quite easy for your vampires to never have to fight on anything but your own terms, at all.