Do you only have Hyborem and your champions? You're making it sound like you've only just started playing Hyborem. If they've got four Angels of Death and you don't have a proper army, you need to wait a bit longer before attacking.
Just hope the Angels of Death don't go around assassinating you mercilessly before then, and build up a force big enough to take them out.
Try to be the one to create the Prophecy of Ragnarok wonder. It requires Way of the Wicked and takes 500

, but on top of providing 8

, +2 Prophet

and 1 Priest, it gives any unit you make in that city the Prophecy Mark promotion. This means the AC goes up by 1 when they're made, and decreases by 1 when they die.
In a production city with the Prophecy of Ragnarok, create a lot of cheap units and keep them alive.
Alternatively, play the Sheaim and spam Elegy of the Sheaim, which is a repeatable ritual which increases the AC by 5 and costs 600

.
Failing that you'll just have to burn cities, look for special events and produce certain evil heroes.
The only modifier in question is a 2 diplomacy modifier between them for the different alignment. I don't think Good nations have a modifier against members of the Undercouncil. 2 can easily be compensated by other factors.