Typical of nuRPGs, best fun is at the at the low levels.
And as it typical with newer games you just need to raise the difficulty higher and higher until you find the sweet spot between fun/challenge. For me it usually highest difficulty before the last, because actual highest difficulty often have some annoying mechanic like double hp/damage that wrecks indented balance and encounter design.
It worked pretty well for me with all new RPG I recently played - King Arthur, Solasta, Dangeon of Naheulbeuk. Well, Solasta is kinda easy but x2 enemy hp would make it's late game even more boring, but other two were consistently challenging throughout the game on default max difficulty.
And it is the same with Wrath - first "standard" difficulty where AI is feature-complete is Hard, so that is what I usually play, or custom. I do not not like playing Unfair since with x2 damage it is just too reloady for my tastes.