then suddenly enemy are spawning in all around your guys (including behind them)
Never fight near the enemy spawn point.
Never let your cavalry run wild unless the enemy is more than 90% dead. They will just run to the enemy spawn point and get slaughtered.
Never charge your infantry. Let the enemy come to you.
I can't confirm your results as I don't take many casualties anymore. I used to get slaughtered by spawning enemies. Now I use the spawn system to slaughter my enemies.
The reinforcement system in this game is awful. It's no better than Warband. Magically teleporting enemies is unrealistic and silly.
STAY OFF THE DAMN RESPAWN POINT AND WATCH YOUR FORTUNES IN BATTLE IMPROVE!!!
Maybe some day we will have a game that can take full armies on the battlefield. Today is not that day.