Answering my own question i can think of some answers: demons of this level clearly are uncommon, I think humanity had some really advance weaponry too before the anti demonization gas (corroborated by that mine Lorem had with her), there is the advantage in numbers, this always will a decisive factor in war, demons seemed pretty unorganized too and demons maybe weren't always there, they may be something that happened only recently (i have the impression that there was a mention that demons are there for some time, dont remember).