Regarding a massive attack – I don't really agree with that. By closing ranks and defending skillfully, Haven will crush all these demon units. Haven is generally designed specifically for defense, so I doubt it can be taken by a cavalry rush.
As I see it, the Demon faction should actively use Berserk, enthrall Lilims, and other abilities that remove units from the battle for 1-3 turns, up to petrification. It makes sense to bring only Abyss Lords to the front, but they are unlikely to withstand a large number of attacks.
It's better to shoot, have the Lords attack with their active ability, and summon as many troops as possible (it's important to max out luck).

Барон "Vampires"
Отдай свои прошлые сны,
В сомненьях себя обрети,
В бешеном танце душа кружится!
Не думай, что будет потом,
Здесь и сейчас мы живем,
Стань хоть на миг сам собой, слышишь!