DaveGold, I suggest you try a few WB tests and see what you can find.
Did you give all the pyre zombies a strength promotion? The AI units always have at least one basic strength promotion, usually more. I also suspect that as they get more stronger they have a larger splash which makes them doubly dangerous.
I'm very well practised at fighting these lousy things in game and sometimes beating them, but usually at massive expense when using exotic tactics. A good example was a game where the Sheiam had wiped out a strong Malakim nation and then attacked me directly afterwards. I was fortunately Hippus with raiders and aggressive and could immediately run down the road and capture back a Malakim city. I then handed the city to a neutral nation with closed borders to the Sheiam which forced the PZ on a long route cross country to my cities (which I thought was quite clever at the time). Even though I was able to launch attacks from the neutral city and raid out the roads, mess up the stacks with splashes, and generally have everything favourable, it was still ultimately impossible to take down their stacks. A strong Sheiam nation can produce a lot of PZ. All I could do was bide time until sorcery came through.