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It's actually quite simple...
You take your starting army, use it to capture resources, and then build level 4 creatures. By this point, you can even sacrifice your entire army, because from then on, you'll only be playing with heroes and level 4 creatures.
Sometimes, the standard tactic of using level 4 creatures and heroes doesn't work, perhaps because you don't have the opportunity to build level 4 creatures quickly enough. In that case, either level up a fighter and use them, supported by a mage, to clear everything, or create a strike force from weaker units and protect it and your heroes in battles, sacrificing the rest of your army and support units as needed.
You take your starting army, use it to capture resources, and then build level 4 creatures. By this point, you can even sacrifice your entire army, because from then on, you'll only be playing with heroes and level 4 creatures.
Sometimes, the standard tactic of using level 4 creatures and heroes doesn't work, perhaps because you don't have the opportunity to build level 4 creatures quickly enough. In that case, either level up a fighter and use them, supported by a mage, to clear everything, or create a strike force from weaker units and protect it and your heroes in battles, sacrificing the rest of your army and support units as needed.

Барон "Vampires"
Отдай свои прошлые сны,
В сомненьях себя обрети,
В бешеном танце душа кружится!
Не думай, что будет потом,
Здесь и сейчас мы живем,
Стань хоть на миг сам собой, слышишь!