Core is the maximum I will go because it is the last difficulty before you need to always: Buff before fights, min-max yourself and your companions, play turn based mode at all times and pick an optimized/op class. At that point it is not a RPG anymore, it is a chore
What mods are you using?
I play Challenging/Core because I want the enemy and me to be playing by the same rules, not one side to arbitrarily be doing reduced or bonus damage to the other.
As for the mods:
And the settings for CotW:
For the Half-Celestial/Advanced templates I just used Bag of Tricks to cheat in a bunch of feats (Wings, DR 10/magic, resists to acid/cold/electricity, immunity to poison, natural armor, etc) and change the point buy and ability score maximums during character creation to simulate the bonuses. The fun thing about Bag of Tricks is that you actually can edit your character in the character creation screen before starting the game. It's just that your character won't have a name yet.
I couldn't figure out any kind of sensible way to get the spell-like abilities so I just skipped those. Also, the SpellResistance11CR feat doesn't work for a PC because PCs don't have a Challenge Rating, so I compromised and just gave my character a 25 SR feat. Overpowered at the start and underpowered late game. If I were being really autistic I'd figure out a way to create a feat that added all the necessary stuff at the appropriate hit-die but the last time I tried to mod a game myself was 5-10 years ago and I don't quite feel like putting in the effort to make a mod for a game I haven't even played yet.
Anyway, I beat the Stag Lord at level 5. Almost died. 3 enemies with magic weapons bypassing my DR, got hit several times in a row even though I had 34 AC and Blur, and I think one of them might have crit me. Poisoned wine+one-hit-killing Owlbear+Glitterdust requiring a nat 20 to save against because of my stupidly high Int saved the day, although some of the enemies did in fact roll a nat 20 to save.
Really regretting picking the Occultist archetype for Arcanist. The enemies are so strong the summoned monsters just get one-shot and actually summoning them is expensive as hell in regards to my Arcane Reservoir. Should've gone with a normal Arcanist for the extra Exploits. Pity there's no Brown-Fur archetype.
I really, really wish someone, anyone in my party had an animal companion, but no go.
I've been running into some bugs with turn-based combat in that sometimes I just can't do anything, or there are visual bugs where the green lines and whatnot for moving will show for the wrong character. I have to quickly go to RTwP and back to turn-based to fix it, but this time it got so bad I had to quit to desktop and restart to make the game un-fuck itself. Not sure if this is a problem with the game itself or because of the mods.