Gonna try and explain the Blazblue layers as best as I can, lots of the explanation revolved around
understanding the lore, still in the process of scan collecting for the verse so if some statements or claims aren't fully backed up with scans, I'm still trying to look for them.
TL;DR, Valkenhayn is flat out immune to the effects of Phenomena intervention from a god like
Takamagahara, who can use it against a Low 1-C structure like the
boundary. In the events of Chronophantasma, there's a PI war going on between Takamagahara and Amaterasu, which
affected Valkenhayn,
something Rachel found to be unreal. This is mostly to establish that beings that are naturally immune to said abilities can be affected when other knowledgeable members point out the events.
In Central Fiction, the Embryo absorbs
all realities into itself to create doomsday, which basically is the apocalypse. The
embryo also causes
Phenomena intervention on a daily basis due
to its mere existence causing it as stated in Xblaze. Doomsday goes beyond even the main BB world as Naoto from a
completely different reality was thrown in by Raquel because
their world got destroyed by Doomsday. Same with Mu-12, who was thrown into a
different reality by Noel and had to come back to the main world
via a cauldron, one of the plot points in the series that talks about going through different realities, which atm are all consumed by the embryo. In said reality Mu-12 is in, Valkenhayn
is again affected by it despite his immunities to believe Mu-12 is the end of all existence among many others in said reality without the proper defense, so it extends across infinite realities with the Embryo's influence against every version of Valkenhayn, plus the additional feat of
Amaterasu's mere presence being able to warp the countless phenomena intervention existences into one (Shiro mentioned the Japanese text says countless and not infinite, but I don't have the JP text to verify it atm). Plus with Observation/PI being able to focus on large scale stuff like timelines or small scale events which focus down into specific individuals, something
Rachel herself comments on is something insane on that magnitude of Observation, leads straight into it being the case of "affecting numerous people and focusing into one target for increased power" with increased potency. Especially when in the context a lot of this stems from someone with a natural resistance towards PI in the first place.
So the argument goes like this: Amaterasu (countless) >>>>>>>>>>>>>>>>> The Embryos (infinite) >>>>>>>>>>>>> Valkenhayn (Low 1-C resistance)