That huge stack of dwagons was actually already severely hurt. They had been performing hit and run operations on the coalitions siege the entire turn in hopes that they would be able to do lots of damage, get really injured back but not actually lose any units so that when the next turn started they would all heal back to full. I'm guessing there wasn't a single dwagon in that stack that was over 10% health... hell probably not over 5% (seeing as Parson is good with the mathamancy and could probably cut it really close)