Have it swallow a Decanter of Endless Water. Use "Shout" to turn on the Geyser. Fill Big T with thousands and millions of gallons of water.

Use "Heroes Feast" to feed the Tarrasque every day. It's not evil, it's just way hungry after sleeping for a few hundred years. Keep feeding it until it goes back to sleep. This method approved by the Grandmother's Guild of Thanksgiving Dinner Preparers.

Use any flying ability to get above it. It can't jump very high. Keep dropping stuff that does non-regenerate damage (check with DM - acid?). Or keep taunting it from just out of reach and lead it away from your friends and toward your enemies. The Tarrasque is a wonderful way to depopulate a border region so you can seize it.