No Wishes needed. Knock it out (via allips or whatever), then drown it.
You know, I've been wanting to mindfuck people over this for awhile and today is my day! Specific trumps general, the Terrasuqa's entry isn't ignored in favor of any text found within Regeneration.
The tarrasque is immune to effects that produce incurable or bleeding wounds, such as mummy rot, a sword with the wounding special ability, or a clay golem’s cursed wound ability.
* Mummy Rot
Is a Supernatural Disease with a magical curse-type element that prevents the disease (something regeneration never cured to begin with) from being healed, the curse is removed by break enchantment or remove curse. This example can be said as defining "incurable" as in '
requires a spell in order to recover from' which covers an insane degree of content. As a fact through, this regeneration also covers none-HP related effects.
* Wounding
Is the ability damage example used to back up the Big-T is immune to ability damage, but you've overlooked something.
Ability Damage cured at the rate of 1 point per day through natural healing.
Ability Drain is incurable as is (see mummy rot & curse wound).
<- Why would you say this is excluded?Both are bestowed through attacks and wounds.
Like Mummy Rot, it says this regeneration also covers none-HP related effects.
* Curse Wound
The damage "doesn’t heal naturally and resists healing spells." which sounds like Ability Drain and nonlethal damage dealt by thirst, hunger, or slow suffocation. This would also include Vile damage.
Q: Can the Terrasqua drown?
As you can plainly see the Terrasqua is immune to any effect that produces incurable hit point loss no matter how incurable they are intended to be nor can suffocation outright kill him (only wish or miracle can). By laughable RAW, there is no "4th round" in suffocation rules either. So even if you said suffocate's instant kill still works, the Terrasqua is immune to any effect that would kill him. On his turn his HP recovers from -10 to 30 and then suffers no ill effects for continuing to remain in water in a defiance of logic and real world physics.
Q: Can we use Ability Drain?
Why in the hell would you think you could? Seriously... Did someone draw the line on the word "damage" and completely forget how to spell disease? *shrugs*