@Helios: you see. In the end, everything comes down to playstyle, given items and availability of troops as I said right in the beginning
@madstrike: hence my recommendation to use Icy Thorns. The thing is: the AI always attack the target they can
a) reach with the fewest number of turns
b) take down the biggest percentage of total leadership with.
c) reduce the most incoming damage with.
That will always come down to level 1 units, or the lowest levelled unit you have in your setup because they have the least defense and health, thus ending up dropping fast. That means Skeleton Archers, Lake Fairies, Sprites, Dryads, Bowmen and so on. Therefore: put everything but your tanks behind ice thorns (except maybe a durable melee striker unit but place that out of reach of the Black Dragon) and make sure, with all force, that you have ******* FIRST initiative. It's vital and there is no way around it.
About Ancient Vampires: they are my #1 choice as secondary tanks or as primary tanks on icy grounds (Demon gets defense penalty). And of course my tanks when I play undead only (which I never play out because I always end up adding Inquisitors, Demons, Shamans, Demonesses or Bowmen occasionally in the end). With enough Atk they beome not only killing machines but also sustain themselves easily through the Life Drain ability in bat form so that you can use Time Back for other things.
Absolutely perfect would be if your game had Ancient Vampires and Ancient Vampire Bats because you can have both. You just can't transform them on the battlefield for they will be out of control because you'll have twice your leadership in Ancient Vampires or Bats respectively.
But beware that Life Drain ofc only works against the living. Undeads and plants excluded. And also beware of no retaliation units for retaliation is a big factor in sustaining numbers through Life Drain. I've seen a huge stack of Bats chopped down by a stack of archers, a 2,500 stack of royal snakes and 3k Scoffer Imps in one turn.
The ring is nice but there's usually better stuff than that. iirc it just pushes their max hp which is nice but not what you need. Distance and strategy are your defense and health bar. Not defence and health.