Ah yes... A classic gamer move. Pull and/or gather as many enemies as possible, then clean the screen of them with a massive AoE nuke of an attack.
Only thing missing is the amount of loot drop and exp gain.
That said, I doubt that was enough to kill Dragonlord. At best, he'll probably be out of commission for a bit.
Arc will probably also even heal him when he gets there.
So it is confusing since their dragon designs are basically the same, but the dragon lord in this chapter is the lady one. Two horns forward is the lady one that lives with the elves, Four horns swept back is the male one that was chilling near the hot springs.