Which monster do you like the most in Heroes 5 (level 6)?

It's not that difficult – mind control, blindness, sorrow, weakness, and finally, debuff! And berserk is the most effective.

Berserk won't work against Paladins, and most likely, Isabel's knights will fall before they can cast a spell, so there's not much point in using berserk. It's better to use other spells, like Subjugate, and even better to defend in the castle.