Quinn could sense a large amount of Qi coming off from Erin, after seeing what she was about to do to Peter as well, he had an idea of what had occured, and his time to fight her was now.
Just because one had a large amount of Qi didn't mean they were invincible or the strongest. It just meant they could use large amounts of it, as much as they wanted. A good example was Quinn's MC cells.
Even if he had an unlimited amount of MC cells, it just meant he could use his shadow and certain skills that would take up a lot of MC cells without a lot of worry, not that his shadow would get stronger.
Of course though, the more you had of something, the better ways there were to use such power, and one would even be able to create bigger moves and condense more Qi to create powerful strikes and power up their body, but in this case, Erin hadn't had the time to do any of that and, consumed by power, she was relying far too much on it.
[Nitro Accelerate activated]
