Now we all know Tarrasque is supposed to be unkillable. And it was in 3.5.
But it isn't in Pathfinder because Zombies lose all supernatural qualities of old form. A Zombie Tarrasque wouldn't regenerate (till someone True Resurrects it).
But how do we get a Zombie Tarrasque?
In Pathfinder, he actually does die from save or die spells just regains life 3 rounds after. Meaning you can totally make a Zombie/ghoul/Mummy Tarrasque.
"No form of attack can suppress the tarrasque's regeneration—it regenerates even if disintegrated or slain by a death effect. If the tarrasque fails a save against an effect that would kill it instantly, it rises from death 3 rounds later with 1 hit point if no further damage is inflicted upon its remains"
So in Pathfinder, you use Disintegrate or a death effect: it dies, then you cast Animate Dead or Create Undead (Pathfinder Zombies don't have HD limit).
It is stuck as a Zombie losing all supernatural abilities till someone Resurrects it.
If you cast disintegrate you can't cast Animate Dead (as no body). But you could Create Undead a Ghost.
Drawbacks:
1) that is a huge amount of Onyx wasting
2) It need to fail save vs Death (and survive each round you fail
3) You'll likely fail control of zombie Tarrasque unless you are very high caster
4) What to do with the zombie/undead afterward
5) Enemy Clerics might try to steal it
You can't actually create a ghost. I has to be animate dead, otherwise the casting time is too long. This was something I'd realized before. :)
Although draining its wisdom to be unconscious is totally a thing a witch with a planar-bound animate dream could do. :)
I was thinking that the best thing a necromancer could do against the Tarrasque is curse it to haunted by the ghosts of all it has slain. Whether being haunted by nations and centuries worth of cranky ghosts could take it down would remain to be seen, but certainly wouldn't make its life any more pleasant. At very least they could lure it to a barren plain somewhere where it would continually destroy phantom cities that it had destroyed before, like Godzilla rampaging through 1950s Tokyo on endless loop.