The strongest castle in Heroes of Might and Magic IV.

They can level up their lord to level 11-12, and the growth problem will be solved.

Everything would be easy if it weren't so difficult. Level 11-12 is a very serious level. You can't over-level. But in the end, it's definitely worth it. It's just a bit challenging. I know from my own experience. I always suffer from my love for lords.

But still, there is already an imbalance: growth.

There is no imbalance in growth, of all things. Rather, an imbalance will arise for others. This is Necropolis, after all. Where does his necromancy go (even if he just summons skeletons or ghosts), and if he's a demonologist, then it's over. On the battlefield, growth is guaranteed.

Therefore, Nature can stand up to Death. First, it's not weak either. It has summoning (the Ring of the Elements, and you're almost in control). Second, let's remember the elementals and their properties. Vampires have nothing to eat.
Order is also good because you can diplomatically get anyone you want by traveling around the map. Thank goodness there is such a thing as Calendar Events.