I'm currently approaching Drezen, just did Lost Chapel.
My party at the moment:
1. Hmoyko the Molfar - a Hex Channeler witch with a Shadow patron. Focuses on casting Slumber and channeling. Has Shadowplay, so presumably his future shadow spells will be powerful. Should have put in more Dex, but no one really attacks him anyway. Has both positive channel and negative channel (Mythic).
2. Camellia - default shaman. Her primary task is putting an evil eye hex so that Hmoyko can reliably Slumber everyone. The other big task is to spam skeleton summon. I hear some people use her as a tank, but for me she's just a non-squishy caster.
3. Seelah - pure Negro Paladin with a horse, wielding a polearm. Great for doing coup de grace on slumbering characters because of her huge reach. Smite evil is also extremely useful against tanky characters.
--- Ottmar - her trusty steed. Tanks really well. I don't understand why no one targets Seelah and instead keeps beating the horse.
4. Lann - default monk archer. Deals a good amount of damage with buffs, but a bit uninventive. Gets enlarged by Camellia.
5. Nenio - default wizard. Not very impressive for now, given the low level. Haste is too important though.
The last member is flexible. At first, I had a vivisectionist Woljif, but he is a bit squishy. Now the main alternative is Daeran, who also has a pet bully wolf via Mythic that is surprisingly good at tripping (e.g., tripping the lich, tripping the ghouls). Stacking two channelers also gives good AoE versus large hordes of undead. He can also help Camellia spam skeletons. But now I am considering replacing Daeran with Ember because you can stack Witch Evil Eye with Shaman Evil Eye, giving -8 to saving throws. This would pretty much guarantee successful slumber as well as subsequent coup de grace (since fortitude is also weakened). It could be further amplified with misfortune.
The balancing is very odd. E.g., there's that item that summons 4 nabasus/angels which steamroll any enemy, including that Level 20 stringy otherling and the tough hidden nabasu in Lost Chapel. Theoretically, it is supposed to be a bit random, but really the chance of getting more than 50% of hostiles is tiny. And you can just run away then.
In terms of difficulty, I kept Hard without changing it. It was somewhat tough at first, but it gets much easier when you get the first mythic level.