When you are going to war, you should prepare yourself by gaining useful information about your enemy. First take a look at the opponents city and look for the following things. You can also use the Spy feature to gain extra information.
First try to destroy the Sky Towers because these are the only towers that will target and kill your Gentle Healers. It commonly known that as soon as all healers are dead you will lose your battle.
How to deal with Watch Posts, you have few options