I did it on my second try by rotating to the next crystal after an ogre spawns and tried to kill him on top of the 0% charge crystal. That way if I killed other adds it wouldn't be a problem since I have to get that crystal to 50% too to spawn the next ogre. Went really smoothly, only tense part was the last (first?) crystal because it was already at 60% charge when killing the last ogre.