I know this is just a thought experiment, but I have to say... Better hope your horse never gets bogged down by a steep hill or a thicket in that full hour of riding.No, you can do it at first level. The Tarrasque has 40' speed (140' total movement if it Dashes and uses three Legendary Actions to move 20' each time) and no ranged attacks. Use your starting gold to buy a riding horse (60' movement) and cast Longstrider on it to give it 140' for an hour. During that hour, the Tarrasque cannot close the range to you, and you can cast Acid Splash 600 times. The Tarrasque has +0 on Dex saves but advantage because of Magic Resistance, so it will succeed on its DC 13 saving throw 384 out of 600 times, which means that it takes 216d6 = 756 HP of damage during the first hour, and it has only 676 HP.
Also that it doesn't get tired.
And that the Tarrasque never throws anything at you.