He used multiple Aspect powers at the same time, contending with the various Aspects fighting in tandem against him, basically countering them at every step by using their opposing power against them and still having the trump card of the Earthwarder's power where Neltharion wasn't present to complete the unity of the Aspects. Chromatus could quite literally do five things at once, despite being a single entity. His immense level of power and the fact that he could wield Aspect-levels of power in the same way the dragon Aspects could is what made him such a force to be reckoned with.
As for the TBC quest, I assume you're referring to Obsidia, a quest mob that's flagged as a group-level challenge that suggests a 5-person group take down. Canonically speaking, she also doesn't die in TBC, as you encounter her again in the Twilight Highlands during Cata, where you again have the aid of the Red drakonid Baleflame to assist you with finally killing her.
I'm not claiming you said that the dracthyr have the power of the Aspects, either; I mentioned their lower power relative to full dragons and the Aspects to further differentiate the dracthyr from the later Chromatic and Twilight flights, and from beings like Chromatus and Ultraxion that *did* have power akin to the Aspects. We do know where the dracthyr's relative power level is as well, we know it by virtue of the fact that they're slated to be a playable race in the game. They're power level is roughly equivalent to that of the rest of the mortals that comprise the playable races and classes - possibly slightly higher lore-wise owing to their unique nature, but certainly comparable, and exactly the same from a gameplay standpoint.