Hidden troops have to be hidden from the deployment stage. Its not so good hiding them during the battle. At least one unit of the enemy always seems to remember where you hid a unit during the battle.

Phalanx troops cannot hide in woods in Phalanx mode...so to deploy them hidden you take them out of phalanx mode. Often the first command they will get as you lure the enemy general to these hidden troops is to turn on phalanx mode.

Do we all know about using spies to open city gates. More spies means increased chance. With 4 entrance points to the AI city you can often get troops in with few losses. Then on stone walls you send them up on the walls to capture all the gateways and towers....which then fire on the enemy.

Egyptian Chariot general has scythed rear wheels. Get AI generals and cavalry to chase it. Keep going but slow/swerve side to side so the persuers catch up with the rear of the generals chariots...and down the enemy will go. Fire at will also means they are being shot at all the time.