A THREAD FOR NEWCOMERS!

A tactic called "hit and run": attack, cast a spell (an offensive spell or Berserk), and retreat. Usually used on single-player maps against enemies that cannot be defeated otherwise at the moment, but need to be weakened as much as possible. Repeated until the enemy army becomes weak enough to be defeated in a normal battle.

No. What you described fits within the bounds of fair play.
In this specific case, it refers to attacking unupgraded creatures because there is a bug in the game: when attacking unupgraded neutrals, there is a chance that one of the stacks will contain an upgraded stack. And if you retreat from or intentionally lose to these monsters, the next time you attack the same monsters, their number will decrease by the size of the upgraded monster stack that was in the battle at that time.

During a siege, my cavalry always goes first. Every single round. Even though the opponent has the same cavalry with the same speed. I repeat the same actions, first attacking the city, then the hero on the street - the result is different!

Have you looked at the battle log? Maybe they are just pressing "wait" or switching to defense mode. Because that makes sense when defending a castle, especially when there are built towers or a stack of archers.