I was thinking more along the lines of destroying all their armies and conquerering their entire nation, but sure, they could fly across their territory to get to the caster instead, I suppose, but one alone wouldn't be able to hold all the items they'd need to both survive in ermor's territory (endless bag of wine, an N1 misc item), not suck in battle (ring of regen (misc item) and vine shield at least, both N2 items, along with a fire brand, probably), and to break down castle walls (wall shaker (A3 misc item which you probably can't make) or gate cleaver (E3 2h wpn which you probably can)). Plus you'd probably want to try to give any iron angels you send armor, helmet, and boots that would help them in some fashion as well - say, by boosting their defense to make it harder for the undead to hit them at all, or by giving them quickness (jade armor, W2E1, I don't think your smiths can get those paths?), etc.
The shield of gleaming gold is normally amazing against melee troops, but awe doesn't work at all on mindless units, which is why I suggested the vine shield instead, since that does.
You have assassins, though, so you could have some of those carry bags of wine and gate cleavers and sneak along with the angels (although since they'll have to siege to help bring down the walls, they'll still be vulnerable eventually), while your angels carry actual combat gear. I don't imagine you have any winged shoes to give them to let them fly to keep up with the angels, though.
If you've empowered any smiths with blood magic (this needs B2), you could make a black heart for an iron angel to turn it into an assassin, but it'd need to be made stealthy somehow first, though. There is an item that gives stealthiness, I think, but I don't remember what item it is.