In your opinion, which faction is the strongest in Heroes 5? Let's discuss the advantages and disadvantages of each faction.

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Based on your post, it seems like you believe that the Dark Elves will just sit back and watch while all the other races get stronger. On an XL map, I will persistently search for enemy heroes starting from level 10-12, only diverting my attention to truly worthwhile targets. Even if I don't have the best starting position or development (in terms of time), a well-developed Dark Elf hero at level 12 can easily defeat a hero of any race that is 3-4 levels higher. I'm not considering artifacts here, as most people do when describing the strengths of their factions; I'm simply saying that with the necessary artifacts, a "dark-eared" hero becomes a terror on the map.
P.S. In my opinion, on an XL map, the Dark Elves' strategy should focus not on their own development, but on preventing the opponent from developing (IMHO).

Maybe I'll surprise you, but I largely agree with you, and a low-level hero can definitely take out many dungeon heroes. I think the critical level is 15; starting from that level (if they're lucky), everyone else starts to have enough abilities and skills. And by around level 20, they definitely will. But how are you going to manage to kill everyone before that on an XL map, especially on a random map? IMHO, it's unlikely. But this can only be verified with a random map generator.