I had the same problem - I found out that, if they are yellow-bar ogres, they don't count toward the grimoire score. Only red-bar ogres (found on the easy setting of the Shrine of Oryx mission, for example) will count toward the total. To level that up fast, choose the Shrine of Oryx on the moon, EASY, then get to the ogre before the final room, kill him, then jump off the left side cliff edge and respawn to kill him again. And again. Etcetera.