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Even if there were 7, it still wouldn't be very weak. These are level 6 units, after all. Although, when playing against the AI, I usually train monks because they are more useful when attacking a castle. But when playing with friends, our final battles always take place in an open field. However, when I play as Warlocks, I would be happy if one of my friends agreed to wait for me in their castle. When playing as Humans, I try to level up a hero who specializes in training troops (skilled counter-attack, experienced commander, troop gathering).
The longer the game lasts, the stronger the army becomes. For Sorceresses, the growth of magical power is limited. The longer the game, the harder it is for the hero to gain new levels.