Ran the very last session of Jade Regent, I quite like books 5 and 6 as written but inserting a kaiju battle while the opposing armies fight was a pretty epic way to end this campaign.
The Five Storms (down to Three Storms for the final battle -Jade Regent, Oracle, and Anam) had control of the Tarrasque. The party after completing the quest at the Emperor's Isle were also blessed with the ability to summon a CR 20 Imperial Sovereighn dragon using the Amatatsu Seal.
So the final battle occurred outside Kasai triggered by the summoning of the Tarrasque to stomp out the rebels.
It more or less eliminated the walk thru of the palace in book 6 but I inserted some challenges around the army and included the Raven Prince attempting to assassinate Ameiko on the eve of the battle. It made for a more fluid last half of book 6, my group seemed to enjoy it.