Battles with neutral creatures.

leveling up several heroes doesn't take much time; you just need to play on online maps where either a tavern is located close to you or by the end of the first week, you already have two castles, and a tavern is located near the second one. Then you'll have level 3 heroes, and after a short time, you'll have level 4. In general, on almost all online maps, it's possible to build level 4 heroes by the beginning of the second week of the game. Then you'll have at least 3 heroes, and leveling up will become easy.