Dig a really big pit. Obtain a large animal carcass. Put lots of spikes on the bottom of the pit. Have lots of men covered in mud to hide on the bottom of the bit, around the spikes, with unlit torches in their hands. Then cover the pit with leaves and branches so it looks like a natural part of the landscape, and put the carcass in the middle of the pit's covering.
Wait until a griffin comes along. It'll be so excited to see the large amount of meat, it'll go away and get its entire family to come feast with it. The weight of the entire family gathered around the carcass with make the pseudo-flooring give way and they'll all fall on the spikes. Then the hiding men can light their torches and burn the griffins to death. Obviously the men themselves won't catch on fire, because they're covered in mud.
And that's how you deal with a griffin infestation.
