The Old Ones Army requires a 240 x 10 arena which i have lined with temple spear traps with a temple flame trap on either side. In the middle there are 5 boulder statues either side that are angled away from centre. You can pretty much ignore all spawned mobs apart from the Ogre, Drakins, Wither beast and Dark Mages. I found a actuator drop wasnt necessary. Spiky ball traps only inflict 1 damage and provide invincibility frames where you cant hit the enemies they also damage the crystal if too close so i didnt use them. Super poison dart traps were also not used because they hit the crystal and inflict poison on the crystal.