Why can't an overloaded Counterflux beat Pyromancer Ascension?
Counterflux works against a singleton Pyromancer Ascension. With multiple active Ascensions, the copied spells don't go on the stack all at once, so Counterflux can't hit all of them.

I found the answer in the comments section of that article:


  Play echoing truth target something, both ascensions trigger.
  First ascension trigger resolves, puts an echoing truth copy on top of the stack, so this copy will resolve before the other ascension trigger resolves.


So to show the stack, since it's hard to think about from the wording:

First, cast Echoing Truth. Both Pyromancer Ascensions trigger.

Stack:

PA#1 trigger
PA#2 trigger
Echoing Truth


PA#1 trigger resolves. Stack is now:

Echoing Truth Copy
PA#2 trigger
Echoing Truth


If you cast Counterflux with overload now, it will counter the two spells, but NOT the trigger, because it doesn't counter abilities. So after it resolves, the PA#2 trigger will resolve and create another copy of Echoing Truth. And if you wait until PA#2 trigger resolves, the first Echoing Truth copy has already resolved. No matter what you do, one Echoing Truth will get through.