UPDATE: Added the combinations next to the spells in the index. Great contribution by FireZealot. Kudos to him/her!

For your leeching pleasure, I took my time to painstakingly create this list of combinations. This list is not the official complete list because I did not go through every possible combination/order, but I what I did do was go through every possible combination for every two elements. So what I pretty much did was put any two elements together in every combination possible and filtered out the positive results.

I not take credit for any part of the creation of the game, but I do take credit in idly wasting my time(which was done purposely >>) to create this list for myself and anyone else who's found themselves playing this game.
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Using the list is simple. The name of the spell is followed by the combination(s) in parentheses. N stands for nature, F for flame, S for shard and so on. The combinations are then followed by their description. I have not tested the effectiveness of each spell, and don't really plan to so don't expect that. The descriptions are usually pretty accurate anyway(regarding what the spell does, not the percentage of effectiveness). So if your mission requires lightning, search for the lightning spell and make sure you make the combination before sending your army off to do a quest.

Here's a little categorization I used to try and make it more usable(use ctrl+f to find the one you want).

Healing Spells
-Lesser Mass Healing (NNNN)
-Lesser Healing (FNNF/SNNS/BNNB/CNNC/TNNT/VNNV)
-Quick Heal (NNTT)
-Mega Heal (VVNN)

Melee Spells(only affect melee users)
-Raw Mana
-Flaming Weapons (FFFF)
-100 Bone Swords (BBLL/BBFF/BBCC/BBTT/BBLL/BBVV)
-Bloodletting (LLLL)
-Poisoned Weapons (LNNN)

Arrow Spells(only affect projectiles so that means casters are excluded)
-Acid Arrow (NNFF/NNCC/NNLL/NNVV)
-Bursting Arrow (FFNN/FFCC/FFTT/FFLL/FFVV)
-Freezing Arrow (SSLL/SSFF/SSCC/SSTT/SSVV)
-Haste Arrow (TTNN/TTFF/TTSS/TTBB/TTCC/TTLL/T TVV)
-Heavy Bolts (NNSS)
-Chaos Bolt (NFSC)
-Split Shafts (NNNS)

Caster Spells(only affects magic types)
-Rain of Bones (NNBB)
-Fireshock (FFSS)
-Frost Shard (SSSS)
-Freezing Strike (SSLL)
-Bone Shard (BBSS)
-Magic Bolt (CCCC)
-Death Missile (VBVB/VBBV)
-Lightning Bolt (VCVC)
-Heat Vision (VFFF)
-Fire Breath (LFFF)
-Aura Bolt (CLLL)
-Shadow Beam (BLLL)
-Ice Bomb (SSST)
-Glacial Bomb (NSTC)
-Acid Bolt (NFSL)
-Decay (VLTC)
-Magical Bomb (FSBC)

Undead Summoning Spells(requires one caster)
-Raise the Dead (BBBB)
-The Burning Dead (FFBB)
-The Freezing Dead (SSBB)

Miscellaneous Quest Spells(only work during quests)
-Destiny Field (CCNN/CCFF/CCSS/CCBB/CCTT/CCLL/CCVV)
-Haste (TTTT)
-Nightvision (VVVV)
-Create Light (NFNF)
-Raccoon Companions (LNLN/LNNL)
-Teleporting (TTTV)
-Enhanced Strength (NSBT)
-Soil Elemental (NFSB)

Miscellaneous Keep Spells(only affects the production at the Keep)
-Enhanced Soil (NTNT)
-Enhanced Husbandry (TNTN)
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Raw Mana - Generally considered a defunct spell this enchantment does slightly increase the damage of your melee soldiers.

Lesser Mass Healing(NNNN) - This sppell applies a slight boost to healing to your entire adventuring party, your hero captain included.

Acid Arrow(NNFF/NNCC/NNLL/NNVV) - This spell makes all of your archers shoot arrows that burst on impact releasing an aid cloud in the area.

Heavy Bolts(NNSS) - All of your archer's arrows and bolt will strike with twice the strength as normal.

Rain of Bones(NNBB) - All of your casters can now cast a rain of sharpened bones. This hits multiple enemies but is best used outdoors.

Quick Heal(NNTT) - This spell makes all of your adventurers heal at a faster rate, which will save many of them from death.

Flaming Weapons(FFFF) - This spell enchants your adventurer's melee weapons with flames causing them to burn anyone they attack. They do not emit light however.

Bursting Arrow(FFNN/FFCC/FFTT/FFLL/FFVV) - This spell makes all of your archers shoot arrows that burst on impact, releasing a small explosion of flame.

Fireshock(FFSS) - This allows all of your casters to throw bolts of flaming fire at their foes.

The Burning Dead(FFBB) - A caster in your group will summon burning undead to fight for them.

Frost Shard(SSSS) - Your casters shoot a razor sharp blade of ice as a projectile.

Freezing Arrow(SSLL/SSFF/SSCC/SSTT/SSVV) - This enchantment makes all of your archers shoot arrows that burst on impact, releasing shards of razor sharp ice.

The Freezing Dead(SSBB) - A caster in your group will summon ice-covered undead to fight for them.

Freezing Strike(SSLL) - Your casters shoot a bolt of freezing water. It stuns and foes and freezes some solid.

Raise the Dead(BBBB) - A caster in your group will summon undead to fight for them.

100 Bone Swords(BBLL/BBFF/BBCC/BBTT/BBLL/BBVV) - This dark blessing imbues all of your melee troops with bone shattering strength in melee combat.

Bone Shard(BBSS) - Your casters shoot a long, piercing bone spear from their fingers.((Ouch?!))

Magic Bolt(CCCC) - All of your casters, priests and clerics included, will be able to fire bolts of magical energy at foes.

Destiny Field(CCNN/CCFF/CCSS/CCBB/CCTT/CCLL/CC VV) - This spell creates a dome of increased chance around your troops. As they adventure events will be more extreme for them, both good and bad.

Haste(TTTT) - Reduce the time that it takes for your people to complete quests!

Haste Arrows(TTNN/TTFF/TTSS/TTBB/TTCC/TTLL/T TVV) - This spell enchants all of your archers so that their arrows fly more true, hitting their marks more often.

Bloodletting(LLLL) - This spell enchants all of your soldier's melee weapons to cause bleeding wounds. Enemies, if able to bleed, will bleedout taking additional damage.

Nightvision(VVVV) - All of your troops, heroes included, can now see in total darkness.

Create Light(NFNF) - Your troops will be able to see no matter how dark it gets. Awesome!

Enhanced Soil(NTNT) - With this enchantment all of your fruit trees will frequently produce additional fruit!

Madness(CVCV) - This spell causes enemies of your adventuring party to become insane, attacking each other and not holding their formations.

Enhanced Husbandry(TNTN) - All of your pig and goat farms will produce additional resources!

Racoon Companions(LNLN/LNNL) - This spell conjures a bunch of magical racoons to acompany your troops. They can unlock doors, sneak through windows, and sometimes even disam traps.

Death Missile(VBVB/VBBV) - This shadow spell allows all casters to fire charged bolts of dark energy.

Lightning Bolt(VCVC) - This spell hits multiple targets with electical damage. You must use casters for it to work.

Lesser Healing(FNNF/SNNS/BNNB/CNNC/TNNT/VNNV) - This spell blesses your adventurers making them more likely to recover from small wounds they may suffer in the field.

Mega Heal(VNNN) - This powerful spell protects as many of your troops as possible.

Chaos Bolt(NFSC) - This unpredictable spell enchants your party with a random element. Which one you may never know... until they like, use their weapons. (Note: In victory sheet, shows up as ranged magic)

Split Shafts(NNNS)- This enchantment causes all of your archer's arrows to split into multiple smaller bolts when fired, thus tripling their power.

Poisoned Weapons(LNNN) - This coats all of your soldier's weapons in an enchanted poison, causing enemies to be weakened with every hit.

Heat Vision(VFFF) - This spell allows casters to shoot fire out of their eyes! What?! So badass.

Fire Breath(LFFF) - This spell allows casters to breathe a cone of fire. This does area fire damage.

Aura Bolt(CLLL) - Your priests and clerics can now release a blast of holy energy.

Shadow Beam(BLLL) - Your casters shoot beams of darkness from their fingertips.

Ice Bomb(SSST) - This allows your casters to use magical ice bombs. These things do a ton of area frost damage.

Glacial Bomb(NSTC) - This allows your casters to deal out massive frost damage in large areas

Acid Bolt(NFSL) - Your casters shoot a bolt of burning acid from their fingertips.

Decay(VLTC) - This spell does a massive amount of area damage, but can also destroy some forms of loot. You must send casters.

Magical Bomb(FSBC) - This allows your casters to use magical bombs. These things do a ton of area lightning damage.

Teleporting(TTTV) - Teleporting your troops to the location of your quest will reduce the total quest time. They still have to walk back though.

Enhanced Strength(NSBT) - This spell will allow each of your troops to carry back more loot.

Soil Elemental(NFSB) - This conjures a soil elemental to help your party carry back their fantastic loot!