As I have said in the preview, a guy with a flying carpet and lots and lots of alchemist fire would be enough to kill the Tarrasque.
Or send a single clay golem after it. Big Ts can only run away then.
Actually there are a lot of things in the MM which can defeat the Tarrasque.
*Turns on sending stone receiver to get 11:00 news*
"And in other news, Waterdeep authorities arrested a man who had accumulated 1,000 vials of the controlled substance alchemists fire in his underground emergency shelter. The man was heard screaming about an 'approaching doom' as he was being taken away. Authorities were tipped off by shopkeepers who had noticed a strange pattern of purchases. We will keep you updated in the days ahead."
After being a pest for a while, the Tarrasque will be frustrated, and the clay golem will just get picked up and thrown far away where it takes damage for its fall distance. It is only immune to bludgeoning damage from non-magical, non-adamantine weapons.
Similarly, after being annoyed for a while, the Tarrasque will pick up something big and chuck it at the creature on the flying carpet. Besides which, the Tarrasque is immune to fire damage.
I agree with [MENTION=67017]Astrosicebear[/MENTION]. The only real weakness the Tarrasque has is the DM. It isn't unbeatable, but it requires a well optimized group and a lot of luck or a plot device.