If you are referring to Voodoo's ::edit:: (sorry, I was auto-typing and got Voodu's name wrong, editing it in here now) Voodu's video on soloing Thunderpeaks, there's some slight confusion from Voodoo Voodu over the ability to damage the dragons whilst grouped; the ability to damage the dragons with untyped/bane damage was always there. (All the way back to the old Aura from Divine Crusader that dealt untyped/bane damage to enemies in an aura)
What they used; Word of Balance, is an Untyped/Bane damage spell, and thus ignores the heavy damage resistance on the dragons. Other spells that dealt Untyped damage also worked; including Disintegrate, Cyclonic Blast and Horrid Wilting. However, these spells have saves, and way back when was virtually impossible to land on Raid bosses, and thus was never really used because you were doing 30-90 damage per cast, and the Dragons had some 4-600k health. So casting these spells was highly inefficient compared to Word of Balance that did not have a save and scaled a bit better since the spell change (it was also awful way back when also, but it got improved since the spell pass).
As far as I know, this is still the case. And no change was made for enable this, and when I solo'd it the other day on R1, it remains the case.
So as long you are working with spells that deals untyped/bane (not physical) damage, then you'll be able to damage the Dragons even when they are close together.
J1NG
:: edit :: Corrected auto-typing of Voodoo to correct spelling of Voodu's actual name.
