[Nagot Gick Fel]

Using a (preferably flying and/or quickened) commander with high attack and damage Ratings and an armor-negating weapon usually does the trick. And it's even better if a few nature mages can entangle the Gryphon rider before. Set everyone to retreat after a round or two and try again next month if it failed.

Maybe you could give one of the heroes a Skull Talisman or Moonvine Bracelet to spice things up?