All monsters are divided into tiers from I to VII, which are basically their levels. Also, each faction has its own weakest monsters that do not have a tier. These can be met in lowest-level squads. Monsters get a lot more powerful with their tiers increasing.
All monsters on the kingdom map are grouped in squads. Squads differ greatly by power and can take any level from 1 to 45. In general, there are 4 main types of monster squads:
Ordinary squads are always attacked by captains, while rare squads can only be attacked by your hero. Attacking rare monster squads is the most efficient way to level up your hero.
Heroic monster squads can be attacked with both your hero and captains. The same applies to Strongholds. You can attack heroic monsters in group marches with your clan allies. This increases your chances for success a lot.