For a start I suggest just you forgot to empower anyone, it's a very inefficient way of wasting tons of gems unless you really know what you're doing (eg boosting that mage to Air4 and you have Cons4 and still a lot of Air gems to make Bags/helms booster).
I'm unclear about what you did encounter : in SP I've never seen the IA empower mages, rarely boost them and overall it doesn't have enough of it by far...If you're playing MP well that's life with a better opponent
With Ermor Early you should get as much Augur Elders as you can, using them as artillery with Fire spells (they have access to the Fire Skull item), and research to Ench4 and Conj3 to summon Banes, Wights and Behemoths for example, these units are strong. Give some a Skull Staff to make them up to D4.
Also make some Skull mentors to boost research, give them to Flamen or such that you can recruit anywhere.
Then go Ench5 for Pale Riders, Ench6 to summon the dreaded Banefire Archers, and Conj5 for Bane Lords, those are even stronger and with some equipment can kill many foes.
A Water9 bless on Equites is not really efficient but fun and usable in SP, as they have a good def. Fire4+ is required in this case IMHO as they have a paltry attack (that's weird as they're supposed to be super-elite ?)
Re: Strategy advice wanted
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