If you want hydra kills, pick The Black Garden mission on Mars and ride your sparrow till you get to the gate. Stand far enough back that you don't get warped into the actual Black Garden and kill the Hydra there, and the 4 Minotaur's and 1 Hobgoblin, wait 10-15 seconds (which gives you time to run down and grab ammo and triangles and engrams), then they respawn again, making the guardians have to kill them to be able to warp again, so kill them over and over, it'll take about 3 hours to get the grimoire done from scratch this way. Less if you have 100 or more kills obviously. It's quite helpful if you equip the Ruin Wings on a Titan and use Rocket Launchers to get your kills, and then run down and pick up heavy ammo, and you won't struggle with them walking around and getting away from you. On a side note, Ogre kills can be done on The Chamber of Night mission on the lowest possible level selection (level 6 I think) so the Ogre is not a yellow bar. It's right before you go into the hallway that leads to the room with Telthor. Just run past the Dregs and Acolytes fighting one another, then kill the Ogre, and run back and jump off the side and die. You'll respawn right before the Dreg/Acolytes again. Just kill, die, repeat.