I am working on a character concept that uses non-lethal means to defeat their (living) enemies (non-living creatures get the normal, lethal damage treatment) and was wondering if anyone knows any ways to boost the damage from non-lethal sources, namely via spellcasting.
I am familiar with the typical sorcerer bloodline/mutation abilities that boost damage per die of spell.
A magus can boost nonlethal touch spell damage (e.g from frostbite) with nonlethal weapon damage. A sap, a merciful weapon, one of the ways of making a scimitar or a bludgeoning weapon nonlethal etc. there are many, many ways to increase weapon damage. Clerics of Torag can use the blesed hammer feat to similar effect. Similarly named bullet + merciful spell can benefit from sling, bow or gun buffs. Arcane archers, spellslingers and eldritch archers can attach spells to ranged weapons or projectiles in different ways. Rogues have a feat line designed for nonlethal melee, sap adept/master which might work for an eldritch scoundrel or an arcane trickster - sneak attack itself is another way to boost some spells of course.
If you want pure blasting then there's other options. Caster level boosts. The obvious metamagic feats like empower spell. Tricks to make the enemy vulnerable to fire like a shaman w/the flame spirit using the hex that does do, or a flame kineticist burning away fire resistance. The winter witch PrC can make enemies take extra damage from cold spells. An occultist with the evocation implement can boost their spell damage but they're not starting from a high base; they can also possibly loan that implement to others though.
Highly recommend sap master. It essentially doubles your sneak attack for nonlethal damage, and still gives some more.
My two-hand vital strike power attack furious focus rogue enjoys it very much for earth breakers.
You could use a Druid. Frostbite lasts for multiple charges (1 charge per level) and causes 1d6+lvl non-lethal damage. Once you get Wild Shape at lvl 4, you could turn into a Deinonychus (or anything with 4+ attacks per round) and have multiple Frostbite hits going off every round. Pick up Natural Spell at level 5, and you can recast Frostbite while Wild Shaped whenever your Frostbite charges go away.