True mind Switching a Terrasque

[Brookshw]

I think primordials controlling the tarrasque might be 4E-FR specific. With regards to your first point, if you're in a tarrasque body, I don't see why you couldn't just avoid going into hibernation with a Ring of Sustenance (if you even need that).

First I've heard of this so I'm guessing 4e. 2e it slept for a decade.

[Hellwyrm]

While this isn't technically using true mindswitch, I know rubick has posted a build that starting at level 6, used the power fusion and the power astral seed to combine with a series of higher and higher hit dice monsters, till finally reaching the devastation beetle at 144 HD, then level draining of the hitdie and replacing them with casting levels.

If you didn't want to go to those extremes, you could be a 15th level psion with the powers fusion, astral seed and psionic dominate, then dominate the terrasque, fuse with it, manifest astral seed, dismiss fusion, then commit suicide and come back as terrasque psion.

[Alent]

While this isn't technically using true mindswitch, I know rubick has posted a build that starting at level 6, used the power fusion and the power astral seed to combine with a series of higher and higher hit dice monsters, till finally reaching the devastation beetle at 144 HD, then level draining of the hitdie and replacing them with casting levels.

If you didn't want to go to those extremes, you could be a 15th level psion with the powers fusion, astral seed and psionic dominate, then dominate the terrasque, fuse with it, manifest astral seed, dismiss fusion, then commit suicide and come back as terrasque psion.

The only thing missing here is fusing with an ooze for split (Ex) before it takes wish to kill you.

[Phelix-Mu]

Thread necromancy alert!

[Invader]

It might have already been mentioned but you should convince your player that big T is garbage and being a diety is better

[Endarire]

The feat Psionic Mastery (Complete Psionic) allows you to take 10 on all manifester level checks. That may help.

As for why you'd want to? Because it's cool. Because you can. Because no one else expects it. Because, "why not?"

[Vaynor]

The Red Towel: Thread necromancy.