Note that once we start getting raid events (next event's the first raid event, actually), Mordred runs into a potential complication. Should there be multiple Mordreds in a raid fight, the collection of debuffs won't stack. At best, I think that they just refresh the duration?
You can probably get away with soloing the standard level raids, so this complication won't show up then. But you're not expected to solo the expert level. That's when the game theory comes in and you wonder about other people using Mordred or not.

Andromeda with D'art second to pick up Sniper Shot as an ex skill (to cover B-frame atk/def debuffing) is the safest general option, I think. May not necessarily be optimal in a given situation, but it'll never leave you feeling 'damn, I really should've gone with someone else'.
...although having just said 'pick up Sniper Shot!', I just remembered that 2 events from now, Medusa (who's after Jormungdr, who's next after Typhon) may leave you wanting to save your ex slot for that debuff remover from Vivian.
And for some bosses, you may want Chaos Magic to remove an atk buff. But with Sol around, you should be fine with running Cassiopeia with Sniper Shot as ex in those cases.