Shrine of oryx level on easy, get to the tunnel at the end, when you hear the ogre roar, dont go any further (or you'll hit a checkpoint) Take him out and then jump off the cliff, rinse and repeat ogre farming. Also, valley of kings on mars, a hydra spawns at the portal every 20 - 30 seconds (must kill all the minotaurs as well) There are also five hydras in the Winters Run strike if you would rather farm that. Grind on!