Archaon AI breaks during final battle

First time with the Archaon end game. Playing Nagash, Archaon invades at turn 71. At 94% ward save he's about to attack the Black Pyramid, possibly ending my campaign. But lo and behold, a wild Mannfred appears and whoops Archaon's ass. Final battle: I send Nagash inside and kite the AI outsite into my gunline, where it gets blasted to death. I capture the first flag, the first wall segment to the town center disappears and Archaon bumrushes to the town entrance. Then his AI breaks and I stuff him with bullets with him not even moving, thus averting the End Times. Safe to say I am slightly underwhelmed.