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If my memory serves me right, you need to rush the enemy cities because the demons respawn immediately with sizable armies, and Kirill grows rapidly.
When I was playing, I remember there were a lot of reloads because the demon hero I had just defeated would manage to return three times with an even larger army and attack me while I was heading to his unfortunate city.
Plus, Kirill has a huge army, but if you can't win tactically, you can wait for a weaker player to engage him and draw away his army... but the main thing is that they are taking troops away from Kirill, which means he becomes weaker each time.