I like combat spells that add tactical options and more indepth gameplay so that's what I'll be suggesting.
Fire: Walking Bomb - Create a humanoid of pure flame that can be moved and blows up when destroyed or at the casters will.
Fire: Blazing Dash - Caster dashes forward in a straight line, dealing fire damage to any unit he passes. (Caster is capable of running through stacks with this spell, can be used to penetrate defensive lines)
Fire: Phoenix Flash - Casts only on self. When struck with a blow that would kill you, the caster is instead reduced to 1 HP and the attacker suffers massive damage possibly killing it outright.
Fire: Sweltering Spellward - Cast upon enemy mage. From now on when casting spells he/she suffers a small amount of fire damage.
Fire: Battle Ignition - All allied soldiers detonate upon death, harming all nearby allies or enemies.
Fire: The Burning Dead - All corpses on the field ignite violently, causing fire damage to anything nearby.
Fire: Heat Flash Nightmare - Cast on battlefield. Each turn all units on the field except the caster suffer from fire damage and have a chance to ignite, suffering further fire damage. If a unit dies from this they explode causing massive fire and crushing damage to those nearby.
Water: Water Clones - Create 3 Water Clones of the caster, clones can move and attack but deal very little (1) damage if attacked the clones are destroyed and the attacker is poisoned.
Water: Water Reflective Illusion - Casted upon a unit, when the unit is attacked the attack is negated and the attacker suffers half the attack damage and is poisoned (Only functions once per cast)
Water: Iced Earth - Sprinkles Ice patches on the Field. when moving if a unit touches or lands on an ice patch is moves in on random direction and stops.
Water: Body of Water - Converts the caster to a water form gaining complete immunity to fire and substantial defense to physical attacks. Electricity however causes massive damage. In this form physical attacks by the caster are weakened.
Water: Reflective Spell Shield - Cast on a unit, any offensive spell cast upon that unit will cause a slightly less powerful water attack spell to attack the offender.
Water: Perfect Reflection - Creates a perfect duplicate of the caster that can cast spells/attack etc. The maximum mana is split between the two. If the original is killed the reflection becomes the original, if the reflection is killed the killer is poisoned. (This is a high level spell obviously requiring several ice/water shards)
Air: Lighting Movement - Caster gains lightning movement, can move instantly to any square for 2-3 turns, anyone near a square he portals to suffers lightning damage.
Air: Storm Front - Caster creates a thunder cloud over the battlefield that for X turns randomly applies one of 3 effects to an enemy.
1- Lightning strike, unit takes moderate lightning damage.
2- Tearing Wind, unit loses all movement for one turn and suffers very light slashing damage.
3- Hail, unit suffers light crushing damage and a slight reduction to Atk
(Again a pretty high level spell)
Air: Wind Blast - Caster fire a blast of wind, has two functions depending on whether the target is an enemy or ally. For enemies is knocks them back and they suffer moderate crushing damage, for allies it gives them a one time boost to movement and attack speed.
Air: Shocking Retribution - Cast on the field, whenever a unit dies his killer suffers double the damage of the fatal blow in electricity damage.
Air: Gale Buffer - Creates a realm of hyper condensed wind on each square around the caster, while active (2 turns) anyone who enters is knocked back and suffers light crushing damage and all projectiles are deflected. Projectile spells are reflected, direct effect spells can break through however.
Air: The Blessed Wind - Causes all units to regenerate health and increases their defense for 3 turns.
Earth: Immortal Stone - Makes the caster impervious to all attacks physical or elemental for 3 turns, however he cannot move and suffers a speed penalty after the spell wears off for another 3 turns.
Earth: Clay Spears - Caster can hide 4 clay spears as traps on the field, when units walk over the trapped locations the spears jab upwards doing high piercing damage.
Earth: Trembling Earth - Causes the battlefield to shake violently, all ground units are incapable of movement for the duration.
Earth: Armor of Rock - Cast on a unit their defense increases and anyone that attacks them suffers an atk penalty for 2 turns afterward.
Earth: The Cradle of Life - The caster shields himself with impervious stone, making him immune to all damage. While this spell lasts he cannot move, attack, nor cast spells, but he regenerates life.
(High level defensive spell. High mana costs.)
Earth: The Spurned Earth - High level earth spell, causes multiple effects.
1 - The Earth trembles reducing movement to zero
2 - Spikes penetrate the ground impaling enemies for high piercing damage.
3 - Pits open consuming entire units... Enemy or Ally.