The best modes to get this in are either Defense or Survival, though I prefer Survival as you face much more enemies in a much shorter amount of time. Plus even in Survival if you're running a full party, you can still get 100 kills relatively easily.
With Survival mode, you'll want your health and armor as buffed as possible, if not in amount definitely in recharge speed. Also, having high attack speed is arguably more important than high damage as you will be swarmed with enemies quite often.
I found using Valkyr with the Hysteria ability makes Survival mode almost unfairly easy, as you take no damage (though you can still be knocked down by strong attacks), your attacks will kill most enemies in no more than two hits and you attack very fast.