First: atmosphere. He's a Bhaalspawn -- a fact that significantly affects the beginning of your romance. Shouldn't there be some intra-godchild trash talk about this, maybe some hints/preview of the ToB goings on? Presumably Tashia's told him all about you after being robotized; he should have -something- to say to you anyway. Also, isn't he a vampire or necromancer or something? It would be great if his henchmen thematically connected to these things, which they don't seem to now.
Second: the battle. Obviously the baddies just drop too much loot. (Arilistan himself should have one nice toy, but minor stuff only for others.) But my 17C/16M (I think -- I ended up levelling up one of them during the battle) PC had an exciting but barely-threatening solo run through the cave. Didn't even delve into the emergency spell case or wands... OK, there's a thread talking about how hard this fight was for some, so maybe it shouldn't be much tougher. But it could anyway be more interesting (or maybe I'm asking for an optional 'Improved Arilistan' component

One thing would be to make slave-Tashia as tough as the real one. The CRE file has her as a level 16 mage (not sorc!), which is really odd... And weak. Is it possible to use the real Tashia's level and spell list? This would allow her difficulty to scale to party power -- I doubt anyone would nerf her spell choices just to simplify this battle. I don't know squat about scripting, but I can't imagine it's impossible to go down a checklist when choosing offensive spells (Dragon's Breath->Comet->ADHW->Chain Lightning->the useless stuff that blew up Vex and the guards) or summonses (a Planetar would be... interesting), and of course to Time Stop[+Improved Alacrity] if she has it. This could of course make her tougher than Irenicus, but hey, if -she's- that level, you're going to slaughter Irenicus anyway.
Vex too isn't as strong or interesting as he could be. He's a level 16 sorc which is fine, but his level 4, 5, 6, and 8 spells don't have a single attacking option, while the level 2-3 attacks are pretty weak (ghoul touch? fireball instead of skull trap?). FoD is nice but he should prepare it with Greater Malision (from a scroll?). And if he's not going to assault you with area attack spells, he should at least haste the fighters and summon up some skeleton warriors/Mordenkainen's swords before going to his doom.
OK, maybe that's a bit OTT. The elite guards seem OK, though throwing axes would make them more formidable. They don't seem to use their potions (nor does Arilistan, for that matter). Not sure about the Rakshasan mages, which skeleton warriors made quick work of offscreen while I was tangling with Arilistan. Both are a bit colorless -- again, I'd hoped for some unconventional attacks, like a bunch of level-draining undead or whatnot. (This would be a bit too much like Bodhi, I realize, but there's surely some middle path.)
Arilistan himself could use a boost too. Part of this is because of the two-phase nature of the battle, which (because you can leave the cave at will) can expand out even further into numerous one/two-opponent battles that are pretty easy. Pure fighter opponents need, I think, either magic backup or strong resistances to challenge post-underdark parties, and even if you haven't killed all his henchmen already it's easy to isolate Arilistan by pulling him into the other room. (His AI kept breaking down at this point, making him even easier than he should be -- is this his specials failing to trigger somehow, or some other bug?)
One way around this would be to keep him and Tashia together in the back, to fight together after the henchmen have flopped. With a high-level Tashia this could be amazingly tough (esp. if you give Arilistan a Bhaalpower, like, say, "Focus"

Another idea is to make him a Kensai/Mage...

As you see, her lovechats had me picturing something more like a mini-ToB bossfight (with something new -- a vampire-associated Bhaalspawn and his minions, who might become one of the five except you kill him here first). What I've suggested may be more like the Ascension version of that, but I think ToB sort of atmospherics -- Bhaalplot allusions, trash talking, novel henchmen, even perhaps ToB music -- would add a lot even if the battle elements weren't (much) toughened. Does this strike a chord with anyone?
(While I'm complaining, I'd also like it if there were some journal entries for the "Rescue Tashia" quest...)