Reading back my previous comment I might come across as indignant. This was not my intention
, I should have taken more time on the response. I acknowledge that on paper Elder God Tarrin is less powerful than the Prime Elder Gods.
So the question now comes to if Tarrin still has some entropic quality as an Elder God and
if he overcome the advantages that the Prime Gods hold.
Firstly he has to get at them when they are either in their home outer planes, in Aiyse's domain or possibly in transit between those locations. For Aiyse herself he'd have to confront her, where she is most powerful, in her home outer plane.
Pyrosian Chronicles 3 - Demon's Bane, Chapter 1, Fel wrote:“Fear is more like it,” she answered evenly. “I’m fully restricted to the Outer plane I call home,” she explained. “Other gods may move between planes freely, but we’re not allowed. I can send projections, surely, but those projections aren’t nearly as powerful as the real me. The only time I’m allowed to leave my home plane is to visit the realm of Ayise in her home plane, and nowhere else. All the Elder Gods can visit Ayise, so we can meet and talk in person rather than through projections.”
Secondly he needs to somehow find or create a threat powerful enough to make each Prime Elder God summit to him, or capable of killing them (we know of one). He would need to capture such knowledge without his objectives being uncovered before he can put his plans into action.
Thirdly he'd need some reason to confront them.
Finally the God of Gods would need to not successfully counter his actions.
He could form a similar threat that the Demon Lords do to the material planes. Where confronting him would cause so much damage that all that the Prime Elder Gods of Sennadar would cause everything they value to be lost. Creating enough of a distraction to cause them to hesitate. Giving Tarrin a chance to overpower them.
If the different worlds [material planes] change status then perhaps Pyrosia could somehow become a Prime World?