Hello. The heal spell IMHO is too easily exploited in the hands of a human playeer while the AI will never use it to its full extend. I would propose changing it. Healing needs to be less easy to spam and have some random elements so its not so much better a spell for the human than the computer players. I'd also like to make it so hero units with healing spells are free to do other things during the battle other than heal every turn. I would also suggest creating a healing-type spell for each elemental type as saving a unit from dying has so many compound benefits. Here are some alternate suggestions

(Life)summon:life nymph

2 turns to cast

low hp well endowed female glowbunny that runs around casting healing touch on units nearby. Can heal enemy units. Can be killed.

(Life)summon: holy bush

2 turns to cast

place a low hp shrub that causes units adjacent to regenerate Hp.

(Fire) Updraft

Stokes the inner fire of a friendly unit, healing it but causing burn damage to nearby units, foreign and domestic. May only be cast on a unti under 33% of health.

(Air) Breath of Life

Prevents death and heals unit to 34% of health should the target drop below 1 hp during that turn. May only be cast on wounded units.

(Air) Favorable breeze

2 turns to cast

Causes a thin line of wind to cut its way through the battlefield for the duration of combat. Any friendly units standing in the breezed tiles receive increased regen and protection from projectiles.