My current party is doing so well on Torment it surprises me. They're basically breezing through the game.
first character: Nephil, traits: Fast on Feet, Elite Warrior. This is the workhorse of the party; with Dex, Gymnastics, and Parry he is hard to hit, always goes first, and usually hits on both swings. Higher Blademaster levels, Riposte and Lethal Blow should come naturally to this guy. Also I found melee weapons to be rather effective, but just on a straight dedicated figher.
second character: Slith, traits: Pure Spirit, Divinely Touched. Priest / fighter hybrid (actually the party's main priest), doing even higher damage than the Nephil, while able to take a beating. I've always found hardiness more useful/interesting on a Slith than defense, mostly because you can make him almost immune to fire damage.
third character: Human, traits: Deadeye, Natural Mage. This character is a dedicated archer/secondary caster who can swing a pole and get up to an enemy too. I actually raise both mage and priest spells on this one to some extent so he can fill the gaps in combat.
fourth character: Human, traits: Natural Mage, Pure Spirit. Main caster of the party (no weapon skills). Since both NM and PS boost your Magical Efficiency, you almost never run out of spell points, and you can cast easy spells like Fire Bolt basically for free early on. This one gets the Tool Use skill too since he has some points to spare.
First Aid is a big plus with this party since you will need your spell points.
Note that this party does NOT abuse special skills and right now I'm never using potions or wands, though that might change later.
As far as tactics go, it's all about choosing your ground and abusing the dumbness of the AI, i.e. make enemies run up to you and waste their action points. Also, on Torment, fighting without buffs is a no go (except if monsters really are much weaker), as is splitting your party. A lone character is prone to getting slaughtered in a single round. It's also nice to 'fight for spell energy'. Since First Aid will restore some pell points whenever you killed someone, if you're low on spell points, just take on some monsters without casting spells, and you will come up the better for it.