The first place I met one of these was the boss on level 3-1 Big game. The suggested level for this is on the easiest difficulty 6 but I had no trouble doing it with a level 5 character with a sword upgraded to level 15 or so. You can simply repeat the level 10 times it takes roughly 7 minutes per run. Try to get back attacks on the Ogre as they do around 2-3 times more damage , let the AI distract it for you or try to get it to charge into things and get stunned.