Alright...broken busted

Flame Seigmund, Ostian Counter, Special Fighter, Black Luna. On anyone. Or Forblaze, Black Luna, and Bold Fighter.

You wouldn't need Heavy or Flashing Blade. Regnal Astra has a cooldown of 2, and Vassal's Blade has -1 special cooldown. So you wouldn't actually benefit from the lowered cooldown.

Oh my, is this a challenge? (With a touch of new skills)

Ares
Default Weapon
Black Luna
Future vision
Ostian Counter
Vantage+ (Unit can attack first even if foe initiates combat, no Hp restrictions)
With everyone
Atk Smoke

Or

Reinhardt
Juicy Wave (Atk)
Future vision
Black Luna
DB4
Bold Fighter
Ostian Pulse
Heavy Blade

For Player Phase:
Reinhardt
Dire Thunder
Future Vision
Black Luna
DB4
Double Lion (Quad 100% if at full HP)
Even ATK wave
ATK +3

SF Nino (flying green mage)
Forseti (Backwords despiration with +3 speed)
Galeforce Or Regnal Astra for overkill
Deathblow 4
Double Lion
Even Speed wave
Flashing Balde

Basically guaranteed quad with 58 attack when attacking and on even turns 47 speed. Either activation Galeforce every turn or quad with regal astra activation twice in one attack (which deals about 18 additional damage each)

Ishtar
Odin's Grimoire(prf)
Future Vision
Regnal Astra
atk/spd bond
special fighter
with everyone!
phantom speed