This prophecy is pretty gruesome. On the heals of the first woe, an attack by demon locusts, comes this second one. This time however, unlike during the locust judgment, the unsaved once again begin dying in droves. A total of 1/3rd of all mankind who is alive at this point will die in this judgment. Given that others will also die along the way, both those who are believers and are martyred, and those who are unsaved and die in the judgments, up to half of the Earth's population, about 3.5 billion (an oddly appropriate number given that the Tribulation is broken up into two 3.5 year timeframes) if you assume today's total global population of 7.125 billion.
This army will consist of about 200 million demonic horsemen who will ride upon demonic steeds and be able to kill people with fire, brimstone and smoke. So those who are killed will either die from incineration or asphyxiation, or perhaps both. There's also one other possible way in which they'll die, and that's by the tails of the horses. Yes, they'll be horsewhipped. The thing is, these tails apparently either act or look like serpents and have the power to kill and harm as well. So that may be a yet third way in which these people will die.
So they can die by fire, by suffocation, or by poisonous serpent bite. What a way to go, eh? Again, believers will be unharmed, and the riders will not come anywhere near them. I'm not sure if this would afford them the same freedom of movement that the prior judgment would, but I suspect that it may give them some, and perhaps even chances to witness, although by now that opportunity appears to have completely dried up as the bible says that those who survive this will not repent, nor give up their evil ways, their idolatry, or any of their wickedness. So I think by this time the low hanging fruit has all been picked, so to speak.
(*Do you have a better description, or did you find an error? Please let me know.