" I recently completed a YT Legendary campaign. Since the skill tree on those idiots was so convoluted I had unreliable access to fire arrows and didn't even bother with trebs.
The first thing I learned was that raider units can storm a minor settlement and torch down the towers. It will hurt, but a full stack vs a garrison attacking at one point has the numbers to pull it off.
Second thing I learned is that AI trebs are not nearly as dangerous as player trebs. The AI may or may not have flaming shot and the unit itself will almost always be nowhere near level 10. At max range their fire can even be dodged to a certain degree, negating serious casualties, especially if they have glowy fire prejectiles that are easy to see coming.
On the field when they are attacking their trebs are slow and lag behind their army as cav bait. Unfortunately for the AI they can become so distracted by the suddenly advancing player army in front of it that they forget to take advantage of the player cav units exposed and all alone behind them...
Be careful when relying solely on great archers to win battles. Sooner or later the enemy will have a trait that reduces your ammo count, and you will find your trash melee units being routed after your six Archery Masters failed to kill one Guardians of Heaven caught out in the open in ambush...
Try to avoid fighting battles in snow. It slows everything down and normal tactics like flank with cav can turn into disasters as volleys of arrows rip them apart while they try to run through the drifts. If you must fight in snow, make the enemy tire themselves out under heavy fire charging at you instead of the other way around.
Horse archers (any cav unit alone or behind them really) give the AI fits. While the HAs themselves are not particularly dangerous the AI may send their cav on suicide missions to try to get rid of them, which can lead those cav units out of sight never to be seen again.
Guerilla deployment is your friend. Three generals with it means either the enemy army or the enemy reinforcements come in to an entire army right in their face, and a full rout seconds later. "
Tactician, from Steam