The requires you to kill 50 corrupted plants and then enter a cavern to kill Lyrathos Darkgrove. For killing the corrupted plants you get a special attack/button that instantly kills them in a radius and that you can use every 5 seconds.
However, if you do not enter the cavern after killing the corrupted plants you can keep using this. The corrupted plants gives experience. I reset my exp/meter and ran this for about 15 minutes and stayed around 300k exp/hour. It's not great by any means but I am sure the method can be improved on.