Dark trash is as follows:
Imp: 3 orbs, AoE attack
Guillotine: 2 orbs, single-target attack
Banshee: 1 orb, single-target Charm (no damage, high proc rate)
I killed the Imp first. None of the attacks were particularly dangerous with ATK debuffs active. I countered the Banshee with Poseidon's debuff protect.
Mid-boss: 4 orbs, high-damage single-target attack
I countered this with Gaia. I missed a Guard once and got hit for 2-4k or so.
Overall, Dark was probably the easiest and most straightforward of the elements so far.