Jason_the_Iguana: So if you do want to see what happens next, I'd advice you to just turn the difficulty down to the lowest level. The campaign isn't that good anyway. Not worth getting frustrated over.
piranha1: Well, as mentioned I shrugged and didn't bother finishing it, but anyway, on the offchance I'd want to try it again (with some other class), how would you handle her as say a cleric? (I might want to try one - probably with at least Knowledge as one of his domains, to make up for lack of rogue levels)
Well, you'd probably have more fun playing the expansions with another class. The expansions are a lot better.
Still, to answer your question: a cleric would handle the fight easily. Aribeth is a cleric, you're a cleric, you'll be better equipped, have better spells, and be smarter.
More specifically: with the right defensive spells (Spell Resistance, Negative Energy Protection, Shield of Faith, Magic Vestment) you'll be able to weather her attacks far more easily, with the right offensive buffing spells (Divine Might, Divine Favour, Greater Magic Weapon, Blackfire) you'll be able to beat her up easily, and that's not even getting started on the offensive spells you can throw her way. (Harm kills almost everything with no save. Word of Faith and Storm of Vengeance will probably win the fight by themselves.)
Clerics, simply put, are one of the strongest classes in the game. Cast enough spells (particularly on yourself, before the fight) and there is no battle you can't win.
Don't bother with the Knowledge domain, though. It's utterly useless. If you want Rogue skills take Trickery instead. (Also very good for the Improved Invisibility spell it provides.) Or just take a Rogue henchman.
My favourite Cleric domains are Magic and Air, because these provide a number of nice low-level offensive spells that clerics otherwise lack, such as Call Lightning, Ice Storm, Acid Arrow, etc. Also, Stoneskin. This is not the most powerful option, but I enjoy being able to blast things with magic.
Domains that provide even more buffing spells (Trickery, Plant) are probably stronger.
piranha1: Also yeah, on topic of non-rogue characters, does the rogue henchman work well enough to detect and disarm traps and open locked doors and containers? (the downside would be that rogues are squishy, is it possible to actually play without one and just walk through the traps or something? or any other way of detecting and handling them, with spells or some such?)
They work well enough. It's a bit slower though, since you need to wait for your henchman to disarm them, or give them the order via the radial menu/a shortcut.
Truthfully, locks and traps aren't that important, since almost all NWN modules are designed to be playable without a rogue. You typically don't get XP for opening/disarming them, so generally the answer to your second question is yes: you can just bash locks open with your weapon and rest if you take damage from traps. There are exceptions to this, but as a rule you won't benefit that much from having rogue skills.
Rogue Henchmen are still pretty good to have, though, because it's convenient and because they can deal a lot of sneak attack damage if you're the type of character to charge in first. Also, the AI is a lot better at handling a Rogue than at handling spellcasters.
piranha1: Another question I got about making a cleric (or any spellcaster in fact): if you get 20 class levels, do you get more spell slots since (class) level 21 onwards? Say if I wanted to make a cleric/Champion of Torm (pethaps with four levels of fighter for weapon specialization). If I'd have 20 levels of Cleric, would getting more give me anything other than epic feats?
You do not get more spell slots in epic levels.
You do get: More caster levels and epic bonus feats. Which you can use for Epic Spells, but those are generally... underwhelming, to say the least.
The caster levels shouldn't be underestimated: they make your spells more difficult to resist by Spell Resistance, and make it much harder for the enemy to dispel your buffs. For clerics, this is vital, since without your buffs you're a second-rate fighter.
Also, caster levels increase spell duration and improve the effect of some spells. For example, Ice Storm continues to increase in damage all the way to level 40. (Whereas a Fireball is maxed out at level 10.)
All in all, multiclassing a cleric with fighter or Champion of Torm can certainly be done, and you won't suffer too much from doing so. However, it's not really necessary: The Divine Might spell already sets your base attack bonus to that of a fighter of your level (Actually, with this spell it's BETTER if you have less than 16 base attack bonus, because of a quirk.) and you have so many spells to boost damage that you'll never notice the difference that Weapon Specialisation makes. (For example: The level 1 spell Divine Favour will give you +5 magic damage already. The level 3 spell Darkfire will make your weapon do 11-16 extra fire damage. Next to stuff like that, +2 damage from specialisation pales.)
I generally just keep my clerics single-class and give them the Weapon Proficiency: Martial or Exotic feat to get better weapons. Alternately, you can take a single level of paladin to get the weapon proficiency for free and also get to add your Charisma bonus to your saving throws.