[Nick_K]

I've now got my copy of Doms3 and I've played two SP games with Yomi as one of the AIs. Both with all AIs on difficult, the second also had all set to standard mentality, Aran and Eye maps.
On both games Yomi has done extremely well comppared to most of the other AIs. In the first Yomi ended up taking over most of the map and actually beat me )it turns out I suck with Mictlan, and I forgot to recruit priests). In the second the map ended up split into three between me (abysia), Yomi and Lanka. I've beaten Yomi, but it wasn't easy even with the fire-9 blessing and lots of battlemages.
I don't know about MP, but it seems that the powerful demon troops with their ethereal second forms making attrition low are very good for the AI and the nation does well in early age SP.
I don't play MP, but I'd take claims that Yomi is underpowered with a pinch of salt, although I can see that there are some obvious counters to them.

As for beating them, have you tried the flaming arrows spell? I believe it adds magic damage to your archers.