I went with "DM wings it", but I'm enough off of that to call for description.
My players have shown no desire to play through a minis-wargame type battle. Nor have they shown desire to be armchair generals. So, I actually don't use mass combat rules at all.
I look at the armies involved, and the terrain, and I figure out how I expect the battle would go. I ask the players what they intend to do in the battle, and then create a montage of scenes or encounters that take place within the battle. What happens in those scenes may effect the final outcome of the battle.