While it might be nice to see some 2e guidelines for how to use rank 6+ entities... there's not really a need for it, because no 2e material is designed for that tier. Especially not Geist.... While its not overly difficult to meet kerberos that powerful, their direct action is pretty limited to "enforcing the laws of their realm" - which they just win at, at that point. Anything else is likely to be entirely roleplayed, done through minions, or will reduce their raw power temporarily (as iirc they lose power they leave their realm)...

But if you're looking for some recommendations...