What mixed hero class of the Haven faction do you like?

Moreover, there is no guarantee that the hero won't be affected.

Drink a potion of immortality before the battle, and take it with you (it's available in almost every castle). And why have an army if you're not going to use it, but just protect it? Are you saving up for the final battle? But it will be attacked many times before that battle.
There's no need to accumulate monsters that guard the artifact. Their number is directly proportional to the size of your army.