I now finished the first campaign. Maybe, some people have good strategies & tips they want to share.

While I am nowhere near an expert on HoMM like games, there are a few very basic tips I want to point out:

(1) Fighting with superior forces allows you to keep losses to a minimum. This is why it is usually advantageous to pool most units to a single wielder
(2) This strongest army should constantly be fighting battles and ignore smaller, unguarded goods that are far out of the way
(3) Additional wielders can be used to pick up the smaller, unguarded stuff. They can also be used to ferry units to replenish the larger army
(4) The defenses of settlements are laughable. Don't bother upgrading contested settlements. But do protect your core settlement
(5) The weaker wielders can also be used to outmaneuver the AI. Simply circumvent their armies and constantly raze cities in their backyard. Don't bother fortifying or upgrading them, they will soon be retaken.

Skills of wielders:

(1) Command: This is necessary to field more units. It needs to be upgraded in your main wielder, matching the growth of your settlements
(2) March: Faster army movement is especially useful for your support wielders
(3) Cunning: Having better stats for the first few rounds is really strong in my impression

Magic:

Destruction and Order seems to be best:

(1) Pacify is great to control large stacks of the enemy.
(2) If you can ramp up chaos quickly, chain lightning can be devastating.
(3) Clouded vision can take out ranged troops.
(4) Swapping enemy units can bring their ranged units to the front & scary melee units to the back. Swapping your own units can make ranged units able to fire if threatened by melee.
(5) Sometimes this can also be done by chaos step.

The rule that you get essence per troop instead of number of units is weird. Splitting faery spirits into several troops with 1 unit nets you much more essence than combining them.

I guess the developers wanted to avoid large armies giving too much essence. But in that case I would maybe scale it logarithmically or something similar.

As I've seen some people struggle with especially the 4th mission of the first campaign, it's important to remember that a high command score allows to spread out your units to get more essence. As noted above, essence isn't determined by the total number of troops but how many groups of units you have on the battlefield. This can hugely boost your essence generation.

If you're struggling with getting past the first few fights, one way to do it to safe guard your knights and ranged units at all costs, while spreading out your footmen as thinly as much possible to serve as meatshields. Pin them down, then whittle away at them. Also, only shoot with your ranged units in those fights at close range because of their need to reload.

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Yes, I also had a bit to struggle to win the last mission. I could only do so with "outmaneuvering" the AI.

But now I probably have a better understanding on build orders and skills, so maybe it's not that difficult anymore.

The way I beat it on the first try was that I put all the troops I could get early get on the main hero, then aggressively pushed and explored. I spotted one of their heroes on the eastern part of the map, beat them and then pushed up to find a large base there that I converted. Weaker heroes pushed west took any towns they could find. I then kept a buffer of small towns that went back and forth while I got my army built up and took the other big north-east town. In retrospect, the AI never got to abuse their town advantage that way.

This is an important point, 1 unit troops can be very powerful. Early on you can use for example militia units and they can be used as "blockers" or to trigger counter attacks, for example you can move them to force enemy attacks on them or use them in melee to trigger an enemy counter attack, allowing you to attack that same unit "for free" with another unit.

Later on higher tier upgraded units even on single stacks will grant you their full active hability power (most of the time) + the essense gain, sure it's easy to one shot them but most of the time it's going to be an overkill that coulda been used on a unit with multiple troops.

Yes, that's really weird. A single Tremor apparently gives the same debuffs as 10 of them.

You could apply a random chance based on troop size vs enemy troop size. If they are the same, you have an 80% chance, if they are larger, only 50% or 30% etc.

Same with essence. There could be a random chance gaining essence from a troop based on its size.

I don't like the idea of adding extra RNG to the game, it would also add some hidden math and at the end of the day we would have a lot of RNG moments like "ohh great, I could not get enough essence to cast X spell because of bad RNG, lovely game".

Single stack units casting buffs is not just a plain advantage, for example the enemy can kill them with a cheap spell giving all their units a buff, also having single stack units could mean not having a lot of cheap units in that "slot" making you weaker. But, if you have the extra slots available and no troops to fill them in they can be quite powerful considering how little you have to invest for them, perhaps a bit too powerful (try single stack Fists of Order, lots of fun for the user)

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There are various solutions possible. It's just counter intuitive that a 1 unit troop would yield the same effect as 50 units.

But yes, we already have a lot of random numbers + deliberately weak defenses so some people might get frustrated.