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More an advice question than a Rule question.
Kaiju isn't a template, so I would compare it to a normal version of the creature and try to evaluate its CR using the monster creation and monster advancement rules.
To me the Colossal size seems a requirement, so a large bone devil kaiju seems inappropriate. I think you would have to increase its size too.

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DR 30 epic and fast healing 30 covers the HP problem (unless the party has easy access to epic weapons), BAB can be a problem, depending on the creature class levels.
It all depends on the goal. A gigantic Bone devil that is almost untouchable by physical attacks, and (arguably) magic, that can easily level buildings but has a mediocre attack against medium-sized creatures can be an interesting challenge if the party needs to discover its weakness to overcome it.