Theoretically you could bring down the Tarrasque with a single casting of Ray of Stupidity, at the whopping cost of a second level spell slot. All you need is a way to overcome the SR. As a 5th level Wizard or Sorcerer you could have Spell Penetration and Greater Spell Penetration. Assume they're human and took Weapon Focus: Ray. Assume that they've also got 18 Dex.
So right there we've got a 5th level arcane caster. They've got: 1d20 + 3 to hit with that ray, for a range from 4-23 on their attack roll. Anything over 5 beats the Tarrasque's Touch AC, so any roll higher than 2 is a hit. The Tarrasque also has Spell Resistance 32, which is the big problem.
You've got +4 Spell Pen from feats. Assuming our caster is still 5th level, that means their roll to overcome is 1d20+9, for a maximum of 29, not good enough.
However, if they are 8th level, they get access to Assay Spell Resistance, which for our purposes amounts to an extra +10 to overcome SR on that crucial attack. At 8th level combined with that, we're up to 1d20+22 for the check to overcome, ranging from 23 to 42. A roll of 11 or higher succeeds. Bam. That's it.
All it takes to bring down the 3.5e Tarrasque is one properly-specced Wizard or Sorcerer with one 4th level spell, one 2nd level spell, one ludicrously low roll, and one slightly above average roll. Maybe a casting of Invisibility beforehand to get close enough. That one mid-level caster can encounter-kill the Tarrasque thanks to a broken spell and the beastie's 3 Int. As long as they've brought along a crowd to beat on the unconscious Tarrasque with an endless series of coup de grace attacks, which are all automatic hits and confirmed criticals. If you've got even a single martial character capable of dishing out damage in the mid-40s, every round is a potential instant death if the Tarrasque fails the Fort save.
disclaimer: it's 6:14 AM and I have no idea how stupid I am right now. But yes, theoretically you could kill the Tarrasque with one 8th level caster and a mob of Commoners with daggers and clubs. If you roll a 1 on the d4, it'll take a second casting of the ray.