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Unfortunately, this is a matter of experience. I also had the same problems at the beginning, but I can still give you a few tips:
1. If the "Vision" spell appears in the magic guild, use it before encountering neutral creatures. This will allow you to determine the exact number of enemies.
2. Play on the same maps. You can even play on one map, and the monsters won't differ much from each other when you start a new game in the same location.
3. When playing as Haven, immediately upgrade the archers to crossbowmen so they can shoot twice. Place them at the very front of your army on the global map so that they stand at the very top on the battlefield. Make sure you have at least 4 units; otherwise, the crossbowmen will be in the middle, and it will be difficult to protect them.
4. Immediately hire a hero of the starting faction in the tavern to strengthen your army.
5. If you can't win on any difficulty level except for "Easy," play on "Easy."
1. If the "Vision" spell appears in the magic guild, use it before encountering neutral creatures. This will allow you to determine the exact number of enemies.
2. Play on the same maps. You can even play on one map, and the monsters won't differ much from each other when you start a new game in the same location.
3. When playing as Haven, immediately upgrade the archers to crossbowmen so they can shoot twice. Place them at the very front of your army on the global map so that they stand at the very top on the battlefield. Make sure you have at least 4 units; otherwise, the crossbowmen will be in the middle, and it will be difficult to protect them.
4. Immediately hire a hero of the starting faction in the tavern to strengthen your army.
5. If you can't win on any difficulty level except for "Easy," play on "Easy."