I'm pretty sure they removed those rules, but wasn't the basic rules for a Tarrasque that they were planar-bound and their immortality only stretched to as long as they were within the plane of existence they are connected to... I'm fairly certain the only ways to kill it are supposed to be A) Kill it then use a greater wish to wish it's corpse out of existence or B)Move it to another plane then kill it. I know that Paizo removed at least the latter method, but wasn't that an inherent part of the nature of the Tarrasque.(In fact, I haven't read it in a long time, but didn't the flavour text say that the death of a Tarrasque killed the core of a plane?)
