I got 2/6 demiatmas collected fully (Realgar, Orpiment) and others having at least 1 when i made it to Knowledge 20 in Occult Crescent just by doing the usual CE > Fate > Fate loop. The rest of them i just got from fates in DT. RNG may not always be in favor but by the time you hit 20 you'll probably collect more than half of the needed, and getting the rest from fates in DT zones wouldnt (relatively) take long.