[Psycho]

Here is a tactic against the AI huge armies: castles and disease demons. Make castles with a good defense rating preferably in some chokepoint places. Let the AI come to you and siege them with his huge army. Then spam disease demons to kill of the commanders. Take the surrounding provinces with small jaguars squads (you will try to build those castles in provinces with fewer neighbors) and attack the commanderless army with a scout and watch them retreat into death.