Dark is the only element you can currently field a good amount of debuffs on, Nyarl and Lu Bu do -30% Atk -30% Def. If you need heals for content, bring Sol for -50% Atk. Use Jack Lantern and try to get Dullahan from friendlist, failing that, Jack Lantern or Apocalypse are fine too.
As for Souls, you'll probably want to run Mordred for his debuff resistance debuff and Black Propaganda. Your EX skill is kinda open, so you can either use Asmund's combo- or probably better yet Sniper Shot for -35% Atk -35% Def. Just remember to apply Sniper Shot before Lu Bu's debuff or Outrage, since debuff overwriting doesn't work.