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Regarding hero selection: yes, Slash, you are right. The most important thing is to choose the right hero. The balance of creatures in Heroes of Might and Magic V is more or less decent, but the hero balance is limping. Therefore, the main thing is to choose the right hero for the right map. But there are also a couple of good ideas that I would like to share: 1. Never neglect scouting. If necessary, buy a secondary hero whose task is not so much to defeat neutrals, but to explore as much area as possible. 2. Don't forget about economy: remember that in Heroes of Might and Magic V, each race has a main resource that is spent in the largest quantities. For elves, it's wood; for humans, it's ore. These resources need to be carefully calculated and, if possible, replenished whenever possible. 3. Apparently, Heroes 5 has a factor of player luck, which depends solely on your fate. Otherwise, how can you explain that sometimes, even on the easiest difficulty, the computer throws huge armies against you, while on Hero difficulty, you can sometimes march straight to the enemy's capital in two or three weeks without encountering serious resistance. Therefore, if something doesn't work out, try playing again.
And now about the heroes:
HAVEN: A great choice would be Ellaina, as a specialist in peasants. But only for large maps. On smaller maps, choose Vittorio (ballista), Dugald (archers), but you need to level up Soldier's Luck. Iris is also a very good hero. Her unupgraded griffons have the "Strike from the Sky" ability, which greatly simplifies things when dealing with ranged neutral units at the beginning. It is worth paying attention to Axel (cavalry), Rutger (pathfinding), and Laslo (swordsmen).
SYLVAN: Here, Talannar (battle frenzy + starts with a ballista), Ilfina (unicorns + sanctuary tent) will be useful. Wingael (+ initiative bonus for units as the hero levels up) is highly respected, as is Vinrael (increased experience gain as the hero levels up). Normal heroes are Ossir (archers), Gilraen (blade dancers). It is not worth paying attention to Dirael (the donkey swarm is not a very impressive spell), but I may be wrong. Anwen (+ damage to all SYLVAN creatures as the hero levels up) was especially appealing to me; I think she is an ideal hero for large maps.
DUNGEON: Here, the clear leaders are Vaishan (scouts), Erin (witches), and Iranna (beasts). Special attention: Sinithar (reduced cost of enhanced spells as the hero levels up), suitable only for small maps where there will be problems with mana. Also: Lethos (can cast the Poison spell on one or more units at the beginning of the battle. Not recommended against undead). Clear losers: Sorgall (increases the damage of the dark riders' super ability, but not their own damage. If it were the other way around, the hero could be quite good), Kifrah (some may disagree, but minotaurs in Heroes of Might and Magic V are not very strong), and Irbeth. Her Dark Ritual ability restores more mana than the maximum. But Dark Ritual takes one turn, which on small maps is equivalent to suicide, and on large maps it doesn't matter, because there are almost no mana problems in the later stages.
ACADEMY: Here, the champions are Nazir (fireball), Orah (the time between the hero's turns decreases as the hero levels up), Maahir (restores mana faster), and Nurah (restores mana in battle). Jalib is good against mages and dark elves, as he can deflect spells into the enemy army. All the others are average, but with proper development, they can "make it."
INFERNO: Leaders: Daleb (ballista), Grak (logistics + teleport spell), Nibros (lucky son of a bitch). On large maps, Nimus is good (increases the number of creatures summoned through the Gate by 1% with each hero level). You just need to level up Leadership => Troop Gathering => Gate Master, and also Luck => Luck in Travel => Wide Gate Opening or Logistics => Pathfinding => Fast Gate Opening. Against mages and dark elves: Marbas (+1% to magic resistance for each level).
NECROPOLIS: I haven't played much, so I'm not sure, but I think the best choice here would be Vlad (raise dead), Naadir (temporarily converts dead enemies into ghosts during the battle), and Zoltan (has a chance to block an enemy spell for the duration of the battle).
Regarding Dwarves and Orcs, I don't know. I haven't played them at all, and I don't really like them. But it seems to me that everything I said is more related to strategy than to tactics. So, I will develop concepts for battles with various races. Although I don't believe that it will help much. After all, victory in Heroes of Might and Magic V depends heavily on luck. Not the hero's luck, but the player's luck.