I played it as basium (but without basium), had some small angel-stack out that managed to get rid of the first demon lord but got smacked from some random SoD afterwarts (5 units with a total of 1500xp
)
my next offensive was chalid with 8 angels, chalid managed to get nearly all available promotions, 650xp or something in the end.
i placed a few cities but got them razed when the apocalyptic riders showed, was a bit sloppy not to think of them.
it was amazing to see varn holding a city with 8 units against two stacks of 20+ demons, most of them CR3++, all due to binding light, seems the AI exploits it, too
without chalid the later part would not have been possible, and binding light enabled me to defend against some SoDs that would have taken me out otherwise.
if i play it again i would focus on unlocking special angels, i am not really used to the mercurians and as long as i was happy with the standard angels it never crossed my mind that maybe heralds etc would have been a lot cooler.
i suggest going empyrion/chalid here, followed by angel-techs, at least if you play as basium. keelyn may benefit from a mage-heavy approach etc.