Also keep in mind your immortality - there are only a few enemies in the game that can actually kill you permanently and cause an actual game-over as opposed to simply having you pop right back up again.

There are even fewer of these that you are supposed to defeat, and even fewer still that you need to actually face in combat (depending largely on your stats, particularly Int/Wis/Cha, there are non-combat solutions available to almost every major enemy; the only ones I can think of where you absolutely cannot avoid actual battle are Ravel - who you already defeated to get here - along with the boss fight in Curst Administration which is about four or so major areas from now, and a mandatory one-on-one fight in the final dungeon).

There are also some ways to cheese certain combats, as you can stack the Clot and/or Heart Charms' resistance bonuses by using several at once, and you can thusly give yourself complete immunity to either slashing or piercing damage (but not both at once); a lot of major enemies sport only one physical damage type and this makes these fights a lot easier (as they can then only harm you with spells, and only if they have a chance to cast them - this is why when I went through as a Fighter I chose to specialise in Punch Daggers, with 5 pips in fists you attack so fast that spellcaster enemies pretty much never get a chance to cast at all).

Let me post my stats to make things more accurate/specific. I'm in Baator now, by the way.
TNO (Mage level 11)
STR 12
INT 19 (+2 tattoo)
WIS 15 (w/ tattoo)
DEX 12
CON 15
CHR 19 (+2 tattoo, +2 eye)

Morte (level 10)
STR 12
INT 13
WIS 9
DEX 16
CON 16
CHR 6

Dak'kon (Fighter level 9/Mage level 10)
STR 18 (+1 w/ Dread bond)
INT 14(+1 tattoo)
WIS 13
DEX 16
CON 16
CHR 13

Annah (Fighter level 9/Thief level 11)
STR 14
INT 12
WIS 10
DEX 18
CON 16
CHR 13

Fall-from-Grace (level 10)
STR 13
INT 16
WIS 16
DEX 16
CON 16
CHR 19

Ignus (level 11)
STR 13
INT 19
WIS 8
DEX 16
CON 20
CHR 3

Thank you both for the extra perspective and welcoming me to the site. I've bought over 110 games here now, but this is the first one I've really played and gotten into.

Regarding level grinding, I usually do a ton of it in RPGs I enjoy, sometimes to the point where it breaks the game, like when I got to the level cap on pretty much every character in FFVI and FFX, but here it doesn't really feel like an option. The exp you need to get to the next level is always so high, and the enemies give you so little that I think I'd take hours to gain even one level at this point. And as you said, the results don't feel worth it.

In terms of my equipment, it's admittedly pretty lacking. I rarely ever give anyone other than TNO anything but a weapon, or maybe some rings if I have them already. I've given Dak'kon a tattoo, but only because it was left over when I replaced one that TNO was using. I haven't gone out of my way to find equipment or weapons for my party members, and I'm not entirely sure what I'd even look for if I did. Also the special teeth I got from a quest for Morte disappeared when he was kidnapped. I'm not sure all this would have as much of an effect on their ability as it would in other games, and also since there had never been much of a focus on combat while I was in a position to acquire new weapons/equipment, I never thought I'd need the extra boost.

Stat upgrades?

I don't take advantage of my immortality. I save a lot, especially before I know I'll be in combat, and load every time I die. Part of it is, I'm afraid I'll miss or lose something if I allow a death, especially after Morte's entire inventory disappeared. (I know now that you can just pick it back up before a couple days pass, but I still have issues with it.)

Another way I'm apparently playing this game completely wrong is I'm not buying a lot of charms. I bought a ton of Blood Charms (60) -- I think it was Blood Charms -- in Curst before I left because I knew things were going down soon, but usually they feel a bit too expensive for me to bother with.

Thank you both again, I hope this info helps clear things up.

It's worth to talk to your party members now and then so you can learn some stuff from or about them.

When you level up put points into wisdom (for bonus XP, luck bonus and dialogues) or maybe con (for hitpoints and regeneration speed), the other stats are high enough already.

Dying is no big problem, just make sure your character is the first to die if you loose so you don't have to come back to pick up your companions' stuff.

TNO doesn't drop his inventory upon death; other party members may do this so you can recover their items in case you never learned Raise Dead and can't bring them back. There are consequences to dying (for TNO), but they don't really matter until the endgame, and even then it's nothing too serious. Abusing you auto-revive is actually pretty fun sometimes. I usually just grind the charms off respawning enemies. Also there's the Blood Bridge spell, which is excellent for TNO to use on party members since he regenerates afterwards. And getting any character's Constitution stat to 20 lets them regenerate hit points like TNO does. [Spoiler]
Morte and Dak'kon can gain a couple extra stat point permanently, Strength/Dexterity mainly, under specific circumstances. Your current Wisdom isn't high enough to unlock Dak'kon's upgrade, though (it has to do with asking him to teach you some of his mage skills); you should absolutely try this, maybe on a later playthrough - the Unbroken Circle of Zerthimon is an excellent read. Morte's upgrade requires, I believe, talking to Fall-from-Grace about him and then questioning him about what you've learned, but I don't remember if anything else is needed to trigger it.

This is a lot of very useful information. I'll be sure to keep this all in mind and maybe come back to it if I do end up doing another playthrough, which is likely considering the surprising amount of replayability here. This is definitely turning into one of my favorite games.

It's also definitely worth it to buy equipment for party members. Unfortunately, the main place to do this is in Sigil. By the time you leave for Ravel's maze it's almost too late.

Priority should always be to boost armour class and primary stats. A quirk of the 2nd edition D&D system is that there's a HUGE difference between having 18 and 19 strength, so giving Dak'kon a strength-boosting tattoo can make him hit like a truck. Oh, and if you get those stat-upgrades by talking to him a lot, another tattoo can get his constitution up to 20 which will make him regenerate.

There are bracelets you can buy that act as armour, but can only be used by mages. These are good for a Mage TNO, as well as for Dak'kon. (Although he already comes with armour.) You can buy armour for Annah, as well as boost her dexterity with items, which will make her hard to hit. But using her offensively may well be even better. If there are big groups, you can sneak up, backstab one, run away around a corner, hide, rinse and repeat. With the right punch daggers and some strength boosting items, these backstabs can one-shot many enemies. (But without the right items it won't do enough damage to be worth the effort.)

Morte, meanwhile, is your best tank. If you look closely at his character sheet, you'll see he has huge damage resistances, so even though his armour class isn't the best he's really hard to kill. The way to exploit this is to make heavy use of his Litany of Curses ability. You can use it as often as you please, so have at it.

Finally, you should figure out which spells are good and which aren't. Some spells are very disappointing for their level. (Fire and Ice?) Others will kill entire groups of end-game creatures in one go. (Cloudkill!) Missile of Patience is also very good once you reach a high level, though a bit quirky in its implementation.

Oh, and while there are level 9 spells in the game, you're very unlikely to ever reach a high enough level to cast them. It's possible, but you either need to grind XP for hours in the Sigil sewers (which is NOT the point of the game and I really recommend against it) or you need to pull off a certain trick at the very end of the game that will get you a couple million extra XP. (Not an exploit, a story event.) But at that point there's basically nothing to cast those spells on.

So if you find level 8 or 9 scrolls, you're better off NOT scribing them in your spellbook, but just using them as scrolls. You'll only get 1 use out of them, but they're really quite a lot of fun, so don't let them sit in your inventory unused.

Spoiler, and apologies for the slight bump, but a correction is in order: the fight in the final dungeon is neither mandatory nor mandatorily one on one.

[SPOILER] AurelianDragon probably meant the solo fight vs. Ignus/Vhailor, which, unlike the confrontations with the incarnations or the Transcendent One, cannot be avoided through dialogue.

SPOILER: Ah, I didn't end up having either of them in my party when I played; it's possible to avoid the fight by doing that. I tend to forget about it because of that.