That's not true. If you try it and the wizard loses, your next step would be to report back, telling us step by step what you did and exactly how the Tarrasque managed to win.Because I am asserting that the plan WON'T WORK, that's why. They say a 5th level Wizard can solo the Tarasque, and I deny it. If I were to play it out and the Tarasque wins, the other side will just claim that (if I run the PC) I didn't try hard enough to kill it, or (if I act as DM) that I rigged things to ensure a Tarasque victory. The only way to persuade them that their strategy will not work is for THEY THEMSELVES to play it out and see that I'm right... otherwise they won't change their minds, see?
Then we'd tell you how you did it wrong, and you'd be welcome to take that knowledge and try again.