1728 First Level Clerics v. 1 Terrasque

A large enough group of level 1 Clerics might be able to take down a Tarrasque with cantrips alone. Sacred Flame the only cantrip that doesn't require the caster to make an attack roll, is of a damage type the Tarrasque is not immune to, and has the target make a Dex saving throw; the only saving throw is has no modifier in (+10 Str, +10 Con, +9 Wis, +9 Cha, and +5 Int).

Assuming each caster has a spell save DC of 15 (meaning they each have a 20 Int score), the Tarrasque would have to roll a 16 or higher (25% chance). The Tarrasque gets three free passes and has advantage on all saving throws from spells or magical effects. That means it has a 43.7% chance of passing the save.

Sacred Flame have a range of 60 feet. A Tarrasque is a titan with a size of at least 20x20. Assuming it's that size, that would mean that only 864 medium or small sized characters could fit around it and still be within range. Two medium or small characters could occupy the same space, so that would be a total of 1,728 characters within range.
Even if each caster dealt minimum damage (1 on a 1d8) and even with a steady 43.7% chance to pass, the Tarrasque would take ~755 damage. (1728 * 0.437 = 755.136). A Tarasque has an average of 676 HP, so this would be enough to kill it.

However ... The Tarrasque has a little thing called "Legendary Actions". It has three legendary actions it can use at any time at the end of another character's turn and regains all spent actions at the start of its turn. The Tarrasque can spend one legendary action to attack (making either a claw or tail attack) or move (at half speed); it can also spend two legendary actions to make a bite attack or use its Swallow action. But the attacks aren't what you should be concerned about. And it's incredibly likely that, seeing all these level 1 scrubs around, the Tarrasque would just leave and go elsewhere. Even at half speed, the Tarrasque has a speed of 20.
Let me paint the scene:
Cleric A casts Sacred Flames on the Tarrasque. Succeed or fail, the Tarrasque takes a legendary action to move 20 feet forward, trampling 56 Clerics to death and making 240 Clerics now out of range to attack.
Cleric B casts Sacred Flames on the Tarrasque. Succeed or fail, the Tarrasque takes a legendary action to again move 20 feet forward, trampling 56 more Clerics to death and making 432 more Clerics now out of range to attack.
Cleric C casts Sacred Flames on the Tarrasque. Succeed or fail, the Tarrasque takes a legendary action to once again move 20 feet forward, trampling 56 more Clerics to death and making 624 more Clerics now out of range to attack.
The Tarrasque is now out of legendary actions.
Even if Cleric D is still alive and in range, 168 Clerics are now dead (almost 10% of what you started with) and 1,296 Clerics are out of range (75% of what you started with). All because the Tarrasque walked forward.

Eventually, the Tarrasque will go (after a handfull of Clerics attack while the rest move back into range) and use his Frightful Presence action, causing all creatures the Tarasque wants within 120 feet of it to make a DC 17 Wisdom saving throw. Even with a 20 Wisdom and being proficient with Wisdom saving throws (which Clerics are), they'd each still need to roll 10 or higher. On average, half of them will fail. That's 780 failures (1728 at start - 168 dead = 1560 reamining / 2 for half = 780). 780 characters who are now frightened and cannot willingly move forward to follow the Tarrasque. The Tarrasque will now make five attacks (if it even wants to) and can then still move 40 feet forward. And with it's legendary actions now back online, it can reapeat this cycle ALL OVER AGAIN.