How to get this achievement?

I already did it and im not realy sure if it was this the scenario.
A bunch of people had the red meteors poop, she casts it to drop. While dropping the lead (it was a wipe alot of people died) just randomly casted it, it cleansed the poops and also blocked prokel (like countering him).

Here a KR video: https://www.youtube.com/watch?v=03BYTr-bKT8

I just got this and it has nothing to do with the meteor poops, it's only about Prokel's shadow (the one you counter), hence the name "Light over Shadow"

Whenever you counter Prokel's shadow, Brel gets a debuff for 35s that prevents Prokel from enhancing her patterns.
We got the achievement by casting Gustaven (Ctrl + Z) on a counter pattern while she did NOT have the debuff, setting the debuff timer to 35s.

However, in the KR video from the other comment they get the achievement without counter pattern AND while she has the debuff already (you can see it as the leftmost debuff icon (gold border)). But we had 2 attempts where we cast Gustaven but did not get the achievement, so there has to be some condition at least. It might be something like "Either Breal does not have the debuff, or it has less than x seconds left, then Gustaven refreshes it to 35s" but I'm not 100% sure.