[Spoilers] Gimped myself a bit - looking for help on the final battle

Rei

The final battle caught me off guard a bit. I wasn't expecting it so soon for some reason.

My party is running low on spells and such and I just can't bring myself to rest during Chapter 12 (or the siege on the castle). I COULD reload my Chapter 12 autosave, rest and meta game it a bit, but it just seems wrong to set up camp and take a nap once Caelar runs off into hell and my obsessive-compulsive nature won't allow it.

I could reload to a save before the siege starts... but I'd really rather not. After literally a dozen or so attempts, I thought I would ask around for some tips/tricks from you more advanced players.

My party (all level capped) (playing on Insane with no damage modifiers, but I'm not sure that matters for this fight?)

Half-elf, F/M - Using Sundermaul +3 (my only +3 weapon, unfortunately)
Jaheira
Corwin (only has 20 +3 arrows, 21 arrows of dispelling)
Safana
Neera (has three reckless dweomers - could these be useful?)
Viconia

Viconia has one True Seeing prepared, but with my F/M having the only +3 weapon, it doesn't seem to last long enough to finish the battle. I side with Caelar (who seems to die fairly fast).

I foolishly scribed the Enchant Weapon scroll from the castle on my F/M, thinking it might come in handy during BG2. But now I realize it was obviously intended to help with the final battle.

I have a quick save where I'm prebuffed with a ton of potions during the elevator, but my F/M usually gets dispelled almost immediately despite having a ton of magic resistance (Cloak of Balduran, potion of magic shielding). I'm guessing you can't resist it because it's such high level? I swear I've resisted dispels before, but a dozen or so attempts and no resists seems to suggest it's not going to happen. I feel like if I could prevent these dispels, I would be much better off.

Any tips for me? I've got two hastes, two breaches, several magic missiles and a spell thrust memorized on my mages but that's about it for noteworthy spells. One dispel, one true seeing, one insect plague on Jaheira/Viconia.

You think this is doable for me? I've pretty much given up. My next step will be loading up a save from before the siege and preparing better spells but I'd love to be able to avoid that. Any tips?

Purudaya

The cleric Dosia (I think that's her name) offers her rejuvenation power during the siege - she stays in the back and can be easy to miss. If you don't want to rest, you can load up a point just as the siege ends (there are a few points that allow quicksaves) and seek her out. If you don't want to go back that far, you could allow yourself to rest once at the entrance to Avernus and just headcannon that your party spoke to her.

Fardragon

You can drink potions during the battle you know. I think it's possible to over prepare.

There are some +3 sling stones in a chest just before the entrance to hell, Jalhera could use those if you don't have a +3 staff.

Edit: I just remembered Caelar has a +3 weapon.

Rei

Yeah. Ended up just going back and preparing minor spell deflection and enchanted weapon and he (she? it?) went down pretty fast. Thanks anyway.