find out what it eats and put out a buffet table (The terrasque is only active for what? 24 hours and it only wakes up to eat right?) Figure out how far it could travel in a day (realisticly. Its not just going to start a walkabout, itll feed and go back to sleep) warn the appropriate populace it could threaten, and be sure to tell their leaders that some whack job is interested in waking up a doomsday machine. (this should get more than one kingdom on your side, get out your best little black number and prepare for WAR!)