I started this as an elaboration of @LorenPechtel's answer, but since it was pointed out it would change the answer too much, I instead show the elaboration as a separate answer.
Before doing this, prepare a lot of high-level (at least 13, to use control undead) Wizards or evil-aligned Clerics. Oh, make sure the Wizards have control undead, and the Clerics have Command Undead.
First, encounter a horde of allips. Use Command Undead or control undead to control them. Given the high level of the characters, they will probably succeed. Control as many as possible.
Teleport to the battlefield. Command them to attack Mr. T. Allips have final attack bonus of +4. Normally, its touch attack should inflict 1d4 Wisdom damage, but Mr. T is immune to ability damage, so let's just hope they score critical hit, which inflicts ability drain instead. Allips score critical threat at natural 20, where the attack MUST land regardless of Mr. T's AC. Now, if they score critical threat, Mr. T has AC 40, so confirming critical is nearly impossible, but wait! They are incorporeal, so natural armor do not work against their attacks! Thus, when threat happens, an allip can confirm its critical hit if natural 1 does not happen. This means 4.75% chance to critical for each allip. Furthermore, T's wisdom score is 15, so it takes 6 critical hits on average to completely drop his Wisdom point to 0.
Now look at the battle at Mr. T's side. He has epic damage resistance, so his natural weapons are treated magical. During full-round attack, T makes 4d8+15 (avg. 33) bite attack, two 1d12+15 (avg. 21.5) claws attack, and two 1d10+15 (avg. 20.5) gores attack with +37 to hit, and finishes his full-round attack with 3d8+7 (avg. 20.5) tail slap with +32 to hit. Sadly, allips only have 14 AC, so they are almost naturally hit by T's attacks. Since they are incorporeal, T must inflict 60 damage to kill one allip. Around two or three attacks are enough to kill one on average, so consider that two and a half allips die off on average, per round.
If everything goes as in expected values and averages, then 0.11875 decrement of Wisdom is expected from T for every round. This means approx. 127 attacks must successfully land to T. A simple solution is to have over 127 allips, but since each allip has 4 hit dice, this would require 43 13th-level Clerics, or 22 13th-level Wizards. Actually, eight 13th-level clerics can control 24 allips, and they can land 130 attacks on average, before dying from T. (Explaination: 24 Allips first attack T. T kills two, and wounds one. 22 (with one wounded) still attack T. (Total attack 46) T finishes the wounded, and kills two more. 19 still attack T. And so on...)
Now, T's Wisdom is down to zero, due to ability drain, not damage. Nothing special is defined on T's sheet regarding this, so let's check the ability score rules. From core rules regarding Ability Scores:
(...) A character with a Wisdom score of 0 is incapable of rational thought and is unconscious.
Unconscious. Not dead. As written, Tarrasque can only regenerate its hit points, not ability scores. Furthermore, it cannot regenerate itself from death, because it is not at all a disintegration, death effect, or some other save-or-die. Therefore, Big T will be unconscious there, until some madman decides to cast restoration on this world-slaying monster. Cheers.
If you are done celebrating, spray some Undeath to Death to kill remaining allips, before they start attacking your mercenaries.