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I would distribute the main schools of magic like this:
1) Earth.
- General slowdown. Makes enemy units terribly slow.
- General stone skin. Increases defense, which is actually very good in long battles.
- General shield. Like stone skin, it is essential in long battles. Reduces damage taken by units during direct attacks.
- Earthquake. In fact, if your opponent is a worthy one, they can easily destroy your catapult while defending in a castle... and what to do? The only thing that can save you is an earthquake, preferably at a high level (or cyclops, if you are a Barbarian or simply have them in your squad).
- Meteor shower. A very excellent spell that damages enemies in an area.
- Resurrection. Permanently revives fallen units.
- Portal to town. Teleports to any friendly town with almost no loss of movement speed on the map.
- Explosion. Massive damage is always useful.
2) Air.
- General acceleration. I have a great love for this spell, as I believe that the best defense is offense, so at the beginning of the battle I decide: defense or a quick attack. If I see that I can defeat the army in one go, I use acceleration. By the way, the secondary skill - tactics, can somewhat ease the situation, and the hero's turn can be spent on a more useful spell at the moment.
- General air shield. Perfectly protects units from enemy ranged units in ranged attacks.
- Chain lightning. Hits multiple targets at once - an excellent tactical move.
- Flight. Flying around the map... you understand.
- Dimension door. Moving around the map four times on a grand level in one turn is, to put it mildly, an imbalance. As is the infinite movement between cities when using a portal.
3) Fire.
- General curse. Enemy units attack with a minimum.
- Blindness. At the highest level, the enemy does not respond to the first attack, so the right ratio of speed and a good mana reserve will help you defeat any unit.
- Frenzy. For those who like it, but it can be useful to, at the last moment, if you are sure of a decisive blow, help your squad put all its defense into an attack. If you don't think it through, you can get into trouble, as the defense becomes zero and all enemy units will attack this unit.
- Berserk. Truly an excellent spell. I rarely hear players talk about it, but it is the most "tactical" spell in the game. My truth in Heroes 3 sounds like this: "Don't let the enemy hero make a move, but play your own game, not letting the opponent's units move." Berserk at the grand level can control units over a large area. And it's not even about their mutual attacks on each other, it's about the loss of the enemy hero's turn, as his warriors move randomly and do not obey him.
- Sacrifice. When there is no resurrection, or God did not allow you to develop earth magic, it happens. After a "good battle," at the end, you can kill a squad of your own troglodytes, for example, to revive fallen skirmishers. It's always useful. There will be room for new creatures that can strengthen your army.
4) Water.
- Summon ship. At a high level, summon a ship next to your position near the water out of nowhere, and the probability is 100%.
- General forgetfulness. When no one in the enemy army shoots, except maybe undead, it's nice.
- Teleportation. Very cheap, five times cheaper on a grand level than without the skill, moves a unit to a specific point in the battle. On a grand level, it can move units behind walls. Some clumsy unit behind a wall to the enemies - let them figure it out... 200 ogre mages. Waiting for them to cross the walls themselves is, to put it mildly, a long time.
- Prayer. A good blessing spell that covers several parameters at once: defense, attack, and speed.
- Walking on water. It can be useful when several islands are located within one move, or simply to move between nearby continents. When there is no time to get on a ship and you need quick movement.
Well, that's it. And now take a look.
Of course, Earth. A wealth of skills for battle that will help you in many situations. And... a city portal, how can you do without it.
Air... some spells for battle and quick movement on the map.
Fire. Spells that spoil enemy units, destroying their initiative for a turn (berserk) or several (blindness, depending on the level of magical power). You can also revive with sacrifice by destroying a less relevant unit for you in order to revive a necessary and stronger one. Health is one thing, but skills. I think that in battle many would prefer 25 halberdiers to one archangel. And this is just resurrection in terms of health, but there is also magical power.
As for water, the spells are not very useful if you have the above-mentioned ones from the schools of earth and air. Dimension door or flight will be better than walking on water. And an air shield is sometimes more useful than forgetfulness, so that enemy units do not move to meet you, an excessive accumulation of enemy soldiers on your half of the field is not good.