Well, I have to admit the city was rather extreme, with lots of stone wardens with medic skill. Actually, for such special cases it's more or less the most easy way to add some pyro zombies. I tried this attempt and after that attack, they couldn't recover anymore.
Therefore the strategy is probably more powerfull than I thought. And the city would have been difficult to get for other power strategies too.
In addition, with some air, I could get maelstrom, and yeah it's correct, in principle I can build a whole army of adepts which can spam strength 4 skeletons and upgrad to mages at some point. With arcane lore, I can even get Hemah with OO religion. And after that, I can get arch mages at some point.
I still think that ritualists get their fire ring spell a bit too easy, but that doesn't mean the mages are bad.