The Blender!
Well, I managed to make a spreadsheet and the lowest level you have a greater than 50% chance of killing the tarrasque I reckon is 9th. That's right, 9th level.
It's fairly complicated. First off, you need to be a cleric with the Luck domain, and you need a cohort who is a Wizard5/Fatespinner3. Secondly, you need to spend all of your combined cash (using PC wealth and NPC wealth for your cohort) to buy the following scrolls: 4 Walls of Force, 5 Blade Barriers [Caster Level 17], 1 Miracle (I am assuming you need to buy the 'expensive' version).
Now you're ready...let's begin.
You approach the beast from the flying position (having your cohort cast Fly on both himself and you). Your cohort now takes centre-stage. Casting Haste, he follows us with Solid Fog. This means that the tarrasque is not going anywhere fast, but the real reason for this is to come.
Being Hasted, he now uses the Wall of Force scrolls. He assembles Walls of Force in a triangular shape around the tarrasque: this takes two rounds, but the tarrasque cannot move out of the designated area due to the Solid Fog. The chances of successfully pulling this stage off are 89%.
Then, you step in with the Blade Barriers. This is why this little combo is called The Blender. Since the tarrasque is trapped, he can do nothing as you try to place as many Blade Barriers as you can within the little tarrasque enclosure. The chances of successfully erecting a single Blade Barrier AND of bypassing the tarrasque's SR (remember caster level of 17th) is around 74%. If the tarrasque resists every one, have your fatespinner reroll the caster level check.
Having set up The Blender, average damage from just a single Blade Barrier (17x3.5=59.5) is nicely more than the tarrasque's regeneration rate, so even if only one is successfully mincing, it shouldn't take more than 45 rounds until he's down to -30hps (and the Walls will last a good 90). Once down, use the Miracle scroll: and if you fail caster level, use your Luck domain power to reroll.