The game should be challenging on the harder levels, no question about that. However, having a dice-roll end your game is just stupid. And, too powerful to do anything monsters wandering randomly in the world are exactly like that. If the monsters are static, then there is no challenge at all in conquering the world.
There needs to be challenges the player can choose to accept. The player needs to accept some challenges to succeed in the game long term. Resources and excellent city locations come to mind as rewards from those challenges.
The quests already work like this. You can choose to accept a challenge, and if you do and succeed to overcome the challenge you are rewarded. That is fun.
The monsters do not work like this. Often there is no choice. You get attacked randomly. If you do attack a strong monsters, the reward is often not worth it.
Of course, there needs to be some (balanced!) challenges the player needs to overcome without a choice to avoid the challenge.