In order to remove a dead estate, I have to attack it until the defense drops to zero. The question..

You can attack to do additional damage but it is a set amount that is minimal. I believe each attack drops wall defense either 2 or 4 points I can’t recall which. Troop march size does not effect the damage so send at least 1 cavalry is the quickest and most efficient method. The 30 minute castle burning usually does more damage for the time period unless you have your entire alliance working together using the method I mentioned.