Well you may not have a magic button, but whatever you did, it's working really well

!
For feedback:
I don't care that much if the AI makes a few bad moves with it's legions so much, as the AI never really has elite crack units that it bases it's strategies upon. What I'm really liking is that they seem to be moving their legions out of garrison duty into the open. This allows the hero to level up a few times and utilize the hero promotions. Since the capture chance is low, and most heroes have high vitality the AI is hardly effected by the occasional defeat of it's core troop. So I think whatever you're doing with the legion AI at the moment is in the right direction, and while it would be nice to see the AI protect their legions better, it's better to see them actually fight with them.
For something you might want to work on 'fixing' is the AI's selection of the UnitCombat level promotions. I've noticed that most of the time, the AI's hero's prioritize the Siege level upgrades before anything else. While this may be good for Yue Jin and I suppose Gao Shun, it absolutely cripples hero's such as Sun Ce and Zhang Liao, who have far more powerful promotions available.
One quick fix that would alleviate the problem somewhat would be to weigh the Vigor promotions (as they are usually the best and most overlooked by the AI) so that the AI always selects it if available. Perhaps you could also weigh a few other promotions such as Indomitable and Conquerer similarly.
I guess I have to eventually spoil the fun though, and report a bug... the Chuan soldier promotions aren't working. I haven't checked the other region promotions though, so they may or may not be broken.
Overall though, this patch is great! The promotion rebalancing was done really well, though it would be nice to see a few more heroes start with a core troop promotion.