Wow, that Doomsaying ability.
It would combine well with the
Ember Weaver, which can suppress any mind-affecting immunity for as long as it can stay awake. Mind control
anything!
So ... Cthulhu has a Will save of +33. You're a caster with Dominate Monster, Greater Bestow Curse, the Leadership feat, and a Ring Gate. Your cohort is a 17th level Oracle with Doomsaying. With the Wicked Leader trait, you can do this at 18th level.
1) Get several Ember Weavers, either with Dominate Monster or Simulacrum.
2) Cohort gives Cthulhu -17 to saves, so he only has +16 against your Bestow Curse. With a good casting stat, he should have a decent chance to fail. Give him the "-8 to rolls" one.
3) Cohort Doomsays again, and the Ember Weavers start using their aura. It's only a DC 19 save, but at this point he only has +8 to save, and you can use multiple of them.
4) Cohort Doomsays one last time, and you use Dominate Monster. With only +8 to save, and the mind-affecting immunity suppressed, it's likely to work.
5) Now, glue one side of the ring gate onto him, and have your Ember Weavers sit in a circle around the other side. Have them use the aura in shifts so there's always one maintaining it. Or if they don't need to sleep (unknown), then just have one sit there maintaining it forever.
6) Cthulhu is now your minion.
Q: What about the whole insanity thing?
A: Mind Blank. Ember Weavers can exclude any creatures they want from the aura.