See the description on Kiya's site. I found it more reliable to remove points from skills and/or add unused skill points, then learn new skills from within the game.

Mage spell books can also be bought (I just about maxed Spell Shield and Aura of Guarding this way).
Free skills, and spell books you can buy

Warriors can win any fight with ease. If you drink a shadow potion and use your spin attack near (but not directly on) a creature, you will hit it without turning visible.
If you don't want to use such an exploit, Stun helps once you get close enough to start whacking, since their ability to attack or defend is reduced. Flash Attack could help with getting close.