Hmm, long post now. I'll only make a short answer. Arralen has given some input and I agree at least to a degree.
MR will not be upped. The demons of Yomi are not of infernal nature as much as evil and uncivilized spirits of the wild. They do come from hell, but are quite comfortable living in the mountaineous and volcanic regions of this world. Giving them more MR would not fit. Making them more resistant to banishment is not a good solution either. Excorcism is quite prevalent in the east. Perhaps more so than in western tradition. So increasing banishment resistance (wich by the way is not a vialble option game-mechanic-wise) does not fit my conceptions.
Raising Ghost MR on the other hand is not a bad idea.
It might be combined with another idea that was discussed in the beta: oni flee when turned into a ghost. This would decrease casualties.
I'm not opposed to sacred armed oni warriors, but I haven't any ideas formed atm. I would probably make them summonables.
I prefer lower costs to higher stats. There are rarely swarms of oni atm.
EDIT: hmm, so much for a short answer
