The actual the question here seems less to be about the skill combination than it is about simply which soul to get.

Getting D'art is for getting Sniper Shot (-20% atk/def b-frame). Getting Mordred is for Black Propaganda (overdrive extension). Both are extremely important for any difficult content but who you prioritize first depends on who you have as a soul is simply there to fill the missing skills your team doesn't have. Since you seem to be water, if you have Belphegor, then you can hold out on D'art. Since you said you don't have Ryu-Oh then you'll need to get Mordred. Cthulu, as great as she is, doesn't play much a role in deciding between one or the other but instead having her you can hold off on Joan.


Joan's main skill reduces all damage taken for your entire team (and allies if in a raid) by 40% for 1 turn.