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On the first level, I take the old imps; I think their dwelling is better, and they don't waste turns.
On the second level, definitely new demons – forget about the explosion before the jump! The explosion is really hard to use effectively, but the jump is the opposite.
I'm still not sure about the new Cerberuses; hitting over 2 cells is very good, but their attack has been significantly weakened.
The Succubus tears demons to shreds; their ability often saved my hero or helped preserve more of my army.
The old horses are better because, with a bit of luck, they can deliver a practically unreturned attack.
The new Yamshiks (from the word "yama" – pit) only look good with a surprise attack because they can block archers, and they can also deal with some enemies quite well, but of course, only the old ones.
And the Demon, I think, is equivalent to the new demons; maybe the new Yamshik will work too.