Update: As I said, after 3-4 tries, I managed to win when the knights almost took out the paladins, and then I slowly chipped away with vampires. But it was close; the knights need to be kept at bay so they don't reach the remaining units, because 10 of them can one-shot them instantly. It's very close to being too difficult, but let's say it's acceptable. The Conflux makes the "Holy Word" ability a bit overpowered, casting it constantly. If it weren't for the vampires, I don't know how I would have managed.
Update: As I said, after 3-4 tries, I managed to win when the knights almost took out the paladins, and then I slowly chipped away with vampires. But it was close; the knights need to be kept at bay so they don't reach the remaining units, because 10 of them can one-shot them instantly. It's very close to being too difficult, but let's say it's acceptable. The Conflux makes the "Holy Word" ability a bit overpowered, casting it constantly. If it weren't for the vampires, I don't know how I would have managed.