I got all of mine in Ruins IV and Ruins V, because I figure they have to give people just starting off a chance, I can do those chambers quickly and without potions, and it maximises my CXP per minute. And it did pretty well in satisfying the bonus reqs, 30 foes / 15 challenges, because: three red books! Motivated! 😃 The registry was filled before the task was, fwiw.