Dark seems like fine first choice. As for second...
I will recommend wind! Main reason being that you have Gaia (Gaia is the best girl!).
As for secondary reason... You can easily reach 30% atk down and 15% def down with Cybele and Zephyrus alone. Add Joan to the mix and you have easy 50% atk down plus 40% dmg cut every 3 turns and some regen for 3 turns every 6 turns. Did I mention that Gaia can also tank single target atk pretty well and then heal herself with her attack skill?
Now if only there was some good wind healer... If you take Andromeda or Cassiopeia would change situation to 40% atk down (if you have Jorm or Yatagarasu in sub slot, maybe even both) and 15% def down with 40% dmg cut every 6 turns. I don't think it is bad.
Well... if you don't like wind then you can go with thunder... I guess? They also don't have any good healer and their good points would be (probably) Tyr with her max dmg on stunned enemies (like Siegfried) and Ramiel with 3 chances to charm enemy and atk/def buff for whole team? Thunder seems to be very aggresive element, kill before they can kill you or kill them before you kill yourself.
Ah, and Tyr def debuff is... 10%? Something like that.