And his powers are pretty impressive, e.g. swallowing people into the earth, burning them from the inside, turning a village into rabid zombies, changing metal & stones to wax, teleporting people and rooms around, darkening the sky, spoiling food, increasing vermin, letting the death walk, etc.

Certainly even more horrible things - like natural disasters, water turning into blood and an eternal night - will eventually happen (those things are already seen in Ishy's visions at the DF Social in TGH).

All done to weak, discourage & kill mankind.

If the DO then manages to focus his Powers a bit more and let those effects become less random, no army in the world has any chance against it. He could e.g. simply turn the soldiers' weapons and armors into wax, teleport channelers away, swallow others into the earth or fry them, while the anyway demoralized and starving rest of the army is done by the attacking Shadowspawn forces.

Of course, we still know from RJ that the DO will lose. But it will be interesting to see how Rand achieves it.