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I'll write about Multiplayer, and I'll immediately clarify that the main difficulty there is the Master level (second to last). Although, of course, the difficulty mainly affects the game duration. So, first of all, you need to develop heroes not for neutral creatures, but for Fortress battles.
If you're playing standard maps, without any specific features, then, of course, you should first develop the main magic or tactics skills, and separately, scouting. After that, you assess the situation: if you've collected various armor and consumed many sapphires, i.e., your defense stats are decent (at least 60), then it's more advisable not to worry about combat skills and take the second branch of skills for summoning a specific class (demonologist, monk, heretic, ninja, crusader, etc.). Sometimes, of course, you can kill two birds with one stone – you need a paladin, a fire mage. But if the mage (who is supposed to cast spells in Fortress battles until they're exhausted) has poor armor, then, of course, you take combat skills. But, if possible, only armor and magic resistance – ranged and melee attacks are not needed.
As for those maps where you can rush your opponent effectively, you definitely need to create one so-called "tank," and develop everything in them except ranged attacks. We don't take ranged attacks because it's not a very effective use of precious skill points at the beginning, because, firstly, damage from hand-to-hand combat is greater, and secondly, you still need to keep monsters away from the fragile mage-supporters.
Mage-supporters also don't need combat skills at the beginning.