"With doom upon us, only a mystical master can finally banish these fiery tricksters and harbingers."
DOOM with Magic or Clea Team-up Defeating Deceiver, Lavaheart and Herald 5 times each.
This is now confirmed by myself and several others. I did not do a Limbo run, and only defeated other things incidentally.
Thanks @PrinnyBaal for the clue and @Lokiator for the suggestion.
Thanks @Kitja and @CaptAwesomeJr for confirming the answer.