Running AV right now, been tons of fun. So much fun in fact that I made the possible mistake of indulging my players with too much sidequesting and bonus content when they wanted to follow up on a plotline or interact more with a particular NPC, so they've wound up a bit over-leveled and over-geared for the AP as written.
Which isn't a huge deal, the Elite adjustment exists for a reason, and you can just add an extra monster here and there when necessary. But now they're most of the way though level 9 of the Vaults, and I'm a little worried about the Final Boss.
One nice thing about AV is that the "Belcorra Attacks!" events mean the party can get a taste of what they're up against and plan accordingly. The first of these, on floor 8, went off fantastically - I only let Belcorra stick around for three rounds, but that was enough for the fighter to wind up Confused, he left the thaumaturge bleeding on the ground, and the Wizard crit failed against Feeblemind and had to be carted back to Otari to have his brain fixed. As a lesson that this lady ain't nothing to mess with, it worked a treat.
But they learned it too well, and this last session they had their second bout against Belcorra and it was a whole different story; they were prepared and sent her packing. So now it's HER turn to learn a few new tricks.
Boosting her a level is certainly an option; she's level 12, the party is currently 10 (again, I let them partake of the sweet nectar of sidequests) and they'll probably be 11 by the end, so making her 13 keeps her competitive. But they've also seen a good portion of her spell list for countering purposes, they're all packing Ghost Touch weapons, and they're using strategies like Disruptive Stance. As a GM I'm pleased they're playing smart. As Belcorra I'm annoyed.
But of course she's learning their capabilities from these encounters too. I don't just want to play Rocket Tag and boost her numbers to be scary, the PCs are learning, Belcorra should as well. Can you think of any suggestions to make the final confrontation more competitive that won't just amount to Ghost Steroids?
Well to start I would have their be a level cap on the players so that they can be over leveled for everything but the last floor, or at least everything but the last room. Let her be elite, thats an easy adjustment and the players can end the game a level higher than intended, thats not the big issue.
Lets get one thing out of the way. No bullet sponges. We don't like them in video games we don't like them here. That said consider how much damage on average they seem to be doing throughout the last floor and give her at least three rounds of HP assuming no crits. If the party crits they will understand that the fight wasn't fast they just got a lucky blow.
If the entire party is dog piling her and dealing damage three rounds should feel right to them. Fight will naturally be longer as PCs miss or focus on other targets and that will feel natural too.
If you really need a buff raw numbers only make things interesting where a fight would be one sided. Numbers should be used in service of a dynamic fight. Fortunately Belcorra is a spellcaster. She knows who the PCs are. Adjust her spell repertoire to counter to them. Not completely, its only fair the sorceress should be able to change one spell a week with Retraining. Obviously NPCs don't need to play by PC rules but its a pacing thing. There is a benefit to getting to her faster.
But even if she never learns more optimal spells, she has a lot of tricks with that many spells. If you think there isn't enough add some next rank slots but fill them with Heightened spells not new spells. Her slapping at them should be the last option, so many monsters just slap at the PCs spells add something different every time.
Also it is a big room, play smart and keep her mobile. You said they have good gear so maybe they can fly by now, but my groups never have enough flight and their is always a wisp or spellcaster hanging out by the ceiling which frustrates them.
Play her like she wants to win. If a mistake is made have it be natural
The room has a hazard in the lines. You want to make it more challenging without just putting Belcorra on steroids? Lets buff the hazard. DC maybe 2 higher (like elite) and add another debilitating effect.