How to update 100 iPad's? Last year, I had to update 100 iPad to iOS 8. I did them one at a time and it took forever.

We use AirWatch as our MDM.

Is there a better way to update them all?
Yes. You'll want to look at caching server on OS X to avoid downloading 100 copies of the software image and either ground control or Apple Configurator 2. Both can be highly automated and scripted and will re-enroll the devices into your MDM once the job is done.