Here's the singleton build I finished the game with on Torment:
I basically followed Randomizer's suggestions from a previous thread. Start with some mage spells (1-2 points), some priest spells (1 point) and some melee weapons. First raise melee weapons, hardiness and parry to 10. Then raise priest spells to 12, mage spells to 4-5 for icy rain, spellcraft to 10 and resistance to 10. Finally, finish the game after raising mage spells to at least 9 (for blink, which is helpful in the Adze-Haakai, Grah-Hoth and Hawthorne fights for positioning), and ideally 14 - so you can cast cloak of the arcane with items.
With regards to stats, raise strength until you can hit enemies reliably, then endurance so you can survive, and then focus on intelligence by mid/end-game since spells will become the main source of damage in many fights.
You don't need to buy cave lore/tool use/arcane lore, if you use a little trick, although it may not be for purists. Simply create additional characters with the required skills, then delete them afterwards. I see it as bringing an expert to help you out with a specific task, rather than having a companion. Torment can be hard enough without diluting your build into non-combat relevant skills.