Going for Mordred for the DoT effects is a viable idea, but do note that the atk/def debuff aspect of Outrage is B-frame. If you do go Mordred, remember to use Amaterasu's atk/def debuff before Outrage as it's stronger (20% versus Outrage's 10%). And there's redundancy with Amaterasu's blind.

Joan of Arc is also a solid choice here. A 40% defense cut every 6 turns is handy now that we're seeing bosses with nasty Raging overdrives. Trial by Jury's atk debuff is A-frame, so it'll stack with Amaterasu's debuff. Can't scoff at -40% attack, on top of the +30% defense buff from Amaterasu, then throw on that 40% defense cut every now and then. And the regen/combo buff isn't too redundant; you can cast that when Amaterasu's regen/def buff and Brynhildr's atk/combo buff are on cooldown. Also, Joan can use the SSR fire lance from this event.
If you go Joan, at some point consider picking up Gawain's def debuff as an ex skill. -20% and A-frame.