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I'll try to share my opinion on playing as the Swamp:
At the start, a fast, aggressive start by clearing all the territory around the Swamp and breaking through quickly, since wyverns can be built as early as the 1st week, and in general, all creature dwellings from level 1 to 6 can be built and subsequently upgraded quite quickly and easily. A problem may arise with hydras and their subsequent upgrade, as a significant amount of sulfur is required (on maximum difficulty 200%, of course, not everything is so fast and easy, and it stretches out in time due to the lack of resources).
There are many good and useful heroes: it's probably best to start with Bron (comes with basilisks), Korbak (has logistics or something similar that increases movement distance, plus the naga specialization), or Tazar (increases defense); you could also use Verdish (starting skill is healing), but that will be more difficult.
If the map is small and you won't be able to level a hero to level 20 or higher, it's better to make a pure warrior hero without relying on magic; if a long, drawn-out game is expected, then the hero needs wisdom and expert Water Magic (healing, remove spell, joy, teleport, clone, and a few other spells useful for the Swamp, but they are only useful at expert or advanced levels, as well as the single use of other available spells), and combat skills (defense, attack or tactics, leadership, or something similar), logistics can also be added, if it drops.
It's best to take artifacts that give a bonus to defense and attack...
Overall, the Swamp army is quite strong and can withstand any opponent.
Combat tactics: against random creatures on the map, everything is basically simple—we wait with nagas and wyverns, move the others slightly forward, wait for the enemies to come closer, and then wipe out the enemy squads, preferably all attacking one enemy squad at once. If you're dealing with utopias or such storehouses where the squad is in the middle, there should be no problems at all; flyers attack one squad, while basilisks and gorgons attack another.
Against heroes: this is where it gets more interesting. There are many options, and much depends on whether the hero levels are below or above 20, and the type of opponent hero (mage or warrior). If there are many nagas (specifically their upgrades) and they don't die after their first attack, you can immediately send them to weaken a strong enemy squad or block shooters by attacking a not-so-strong enemy squad. In general, it's best to play defensively: the nagas wait, and the others move forward a few cells (tactics is very useful here, as you can position them this way before the battle starts, and at the beginning of the battle just give defense). After the enemy's first turn, attack with your squads—gorgons, wyverns, and basilisks first target one stronger squad, then the next, or send some to one squad and others to weaken another (depending on the situation). Hydras and gnolls are better sent against weaker creatures (I personally like teleporting hydras into a crowd and attacking 1 to 3 squads or taking out shooters; if there is no teleport, then either into the thick of the fight or, conversely, picking off the weakest). Gnolls in any battle are cannon fodder that will be gone by the end of the fight; lizardmen are simply sturdy shooters...
In any case, if there is such a difficult battle between heroes with substantial forces, by the end, a few gorgons, basilisks, and maybe lizardmen or wyverns should remain (though the latter is unlikely)...
Use of magic in battle: Water Magic—most often it's this that occupies the 3 levels of the Mage Guild in the city, and it's generally the only one useful for the Swamp due to healing, teleport, and remove spell, since you'll often encounter a mage hero casting slow, blindness, berserk, curse, or haste, stone skin, or blessing on their own squads. Then an expert in Water will either heal their own or remove everything from the enemy squads; basic healing, which can be used even by a warrior hero without magic skills and by nagas, will save from single spells... the rest is not so important for the Swamp...
In a fast game, especially on medium and small maps where the hero cannot be brought to level 20 and above (more accurately level 23-24, as it seems at that level all hero skills become expert), it makes sense for the Swamp to have rapid development and a rush, but only not against a good mage hero from the Tower or Dungeon with a set of offensive spells.
P.S. I played as the Swamp offline and online with a friend on random maps with conditions (from 1 to 7 computer opponents). On Rook or King difficulty, there's nothing special (Pawn and Knight are just too easy and boring to play); on Queen, it's problematic to find and collect the necessary amount of resources, so rapid development and capturing the enemy city doesn't work, but on the other hand, the game becomes quite long and one or two heroes can be well-leveled, and the AI pleases with normal behavior in combat.
I don't understand any of your SODs etc. etc., as I have the complete Heroes 3 collection installed; there is no troop experience, and no mods etc. etc., only the original game... in principle, all races are balanced, and playing as any race, you can defeat any other...