The AI has changed many times in this game, and so have the winning strategies to exploit its weaknesses. What I suggest you to do is to start learning in steps:
1. BASICS: learn how to "read" the control bar. this is the most important thing to master. being able to simulate a sequence of turns from both your and enemy troops in your mind is a key to every victory.
2. MANUAL BATTLE: play your battles manually and slowly in the begining till you learn how AI moves and what are its preferences. Sometimes it will target the closes unit in range, sometimes it will target a unit of certain type in range, sometimes it will target a damaged unit and sometimes it will skip a turn if it has no units which it can reach in one turn etc. Move your units accordingly checking for range of enemy troops.
3. AUTO BATTLE: after figuring out how AI moves in certain situations, you can apply that knowledge also when doing autocombats. It is the same AI that moves your units in attack when you click autobattle, identical to the one which moves the defending units. This is the way you will be doing most of your battles in later stages of the game so dont be afraid to experiment especially if you have a strong military and attack/def boosting GBs. This knowledge will save you hours of time every day.
This is a beginners
Guide I have done for my guild long time ago where I explain one basic tactic which I called "Side Hook". I honestly dont know if its still up to date with all the AI changes that have been introduced subsequently. However it explains well the APPROACH to battle tactics one should have in my opinion. It is in italian, but I think it's quite understandable if you googletranslate the page.