As your main is light you can ticket for Sol (Heal + C Frame ATK Down + Dispell) or Raphael (Dizzy + Overdrive -1 + Rage Bar Down + A Frame ATK Down) and since you have Diana you'd only need to run Sniper Shot as an EX on your Mordred.
By the way, notice that Mordred's Black Propaganda does stack with Raphael's Overdrive Orb Reduction, this makes up for a neat combo, I think Raphael Cooldown was 6 turns on that skill, so yay(?
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If miracle ticket there is he should focus on Sol / light Tsukuyomi. There is no point getting Raph since he's got Eros already. Otherwise yes, Raph + Mordred is a very good combo.
Unfortunately yes, Metatron is fairly weak. She doesn't go well with Shingen and take a lot of damage because of her debuff. Hopefully she will probably be the next awakened kamihime on dmm so, wait and see...
The best element for overdrive stalling by far would be water IMO. You have Ryu-Oh for overdrive extension, which means you can free up an ex slot/ don't have to rely on Mordred, you have both Cthulhu and Snow Raphy for orb removal, and the girls work pretty dang well together (Any soul Ryu Cthulhu and Raphy or Mordred/ Ex BB Cthulhu Raphy).
And if you want to go Full Retard(tm) on the orb removal thing, Water also offers Dagon. R with the orb removal.
And allow me to note Mordred's debuff- and a Ryu-Oh with 6% Affliction does not give Ryu-Oh 100% success chance with her Black Propaganda (against neutral). It is high, but not 100%. Just something to keep in mind.