I thought the final fight of SoZ was difficult. I felt underleveled and ended up farming a bunch more random overland encounters to level up before beating it. Also, it’s the only time I had to really full-buff before the fight (I used just about every buff the second time rather just the normal long duration buffs + haste).
If I recall the SOZ setup, it had lots of decent HP spell casters spread out in the back, with awkard geometry confusing your movements, and a high HP / high BAB / high damage monster in the front wrecking your guys if they don’t move.
No real “soft” targets and every enemy can do a lot of damage if ignored, and hard to move around / kite because of barriers & static items.
Most NWN2 fights are hard though only because you’re underleveled or you haven’t figured out some trick (high DR that you can’t get through, spells that you haven’t prepared immunities for). Items are just too good in most modules, and kiting to thin out enemies is also a big advantage to the player.