The discussion about the gross demons made me realize that angels and demons, both of whom are in the final battle of book 3, are not represented in the same way.
There are multiple demons inside each zombie body, which is what gives them the strength to make the zombie appear alive. Not only does this make them powerful zombies, the physical body is what allows them to tread on holy ground without the suffering that would normally go with that.
Angels don't use zombie bodies, so they come to Earth having assumed human form, which they project as solid bodies, which allows them to participate in the battle.
Problem is, demons are fallen angels, so they ought to have that same ability to assume human form without having to enter a zombie body. I dislike the complexity of demons having two ways to be solid, so I'll make it so that fallen angels can no longer assume solid form, a power they lost when cast out of heaven.. That forces them to use zombies if they want to assume a human form.
So, angels can assume human form, which is consistent with the Bible, while demons have to use zombies, which are generally not as powerful as angels, unless perhaps ten demons get together in one dead body to act in unison. I'll put an upper limit on the number of demons that can co-exist in one zombie, otherwise they'd just put 100 demons in each body, allowing them to kick everyone's butt.
Further, demon bodies can be killed, forcing the demons in that body out. Once back in pure spirit form, they can't take human/solid form, so they're almost useless until they find another dead body.
In the final battle, Connor, his cousins (Omega Force), and angels all kill zombies, which doesn't destroy the demon spirits (they're immortal), but takes them out of the fight, which is exactly what I need. Since angels can't be killed either, demons can't win against them. If the battle goes on long enough, zombie bodies will either all be destroyed by angels, or the bodies tire out from exhaustion, whereas angels don't.
This'll allow me to send hordes of demons at boosted Connor, genetically enhanced Omega Force, and the angels, yet the latter three can still kick serious butt. Connor is unstoppable while boosted. His cousins all have similar genetic enhancements as Connor, making them powerful but not invincible.
EDIT: I just realized that, since his cousins have very similar (though less successful) genes, they all feel intense loyalty to Satan, same as Connor did until mid book two. Yikes! I'll have to have all of them volunteer to disable the same switch. As with Connor, they'll have only two years left to live, which is beyond the end of book three. Connor, however, should be close to death since he flipped the switch in book two, two years earlier.