What is the biggest single damage number possible (no LB)

[Bardo]

Unfortunately, neither Power Surge or Foe Requiem work with Dragonfire Dive.
Just like WindBite/Venomous Bite/Flaming Arrow, the tooltips seem to say they apply as elemental damage, but they are really physical damage.
Easily tested in Coil Turn4 against Knights/Soldiers.

DRG's biggest single target hit is Combo'd Full thrust(300potency) + disembowel debuff(10%) + Blood for Blood(30%).
That's about equivalent to a 420 potency attack.

Compare that to Flare (260potency) + Astral Fire 3 (~80%) + Raging Strikes (20%) + [Battle Voice] Foe Requiem (~30%).
With buffs accounted for it works out to something close to a 600 potency attack.
Then on top of that you could get a Thunder3 proc that equates to about a 500 potency attack, and then turn around and Convert + Swiftcast Flare again for another 600 potency.

DRG and MRD can't touch that with a 10ft pole.