Archpawn

I'm assuming that even though the DM pretends to be annoyed, he actually thinks all these shenanigans are awesome and is bending the rules to let them work.

I want to see a campaign where you have to protect an endangered tarrasque from a group of aarakocra that are trying to poach it.

I'm not a fan of the reflect ability. It's just immunity, but a bit worse if the party doesn't know about it ahead of time.

It would be much more interesting if it didn't negate attacks it fails to reflect.

In 3.5, a high level wizard could take it down.

In 5e, you could have a mission to protect an endangered tarrasque from Aarakocra poachers.

The best way I've seen to defeat an enemy without killing it is Flesh to Stone, Stone to Mud, Purify Food and Drink, and then boil the water away. That was more for keeping an enemy from being resurrected, but it would be a cool overkill way to get rid of a tarrasque without using Wish.

That’s true of anything without a fly speed,

And without a burrow speed, and without a ranged attack, and without an ability that lets it ground all flying enemies. Maybe a skilled DM could make it work, but in other editions it wouldn't have been an issue.

Though the other problem is that you can deal limitless damage just by dropping sufficiently many 100 pound boulders. In 5e, they got rid of damage from falling objects, but you just need to drop enough creatures. Or ignite enough horns of gunpowder with a single Bonfire.

I've been thinking it would be cool to have a campaign set after the town has gotten smaller. You go on your first mission to fight a cave full of kobolds or some such, there's an earthquake that blocks the exist, and you have to fight through it and escape before later tremors cause it to cave in. It's fairly standard, until you leave, and find out what was causing the tremors. At some point decades or even centuries ago, the rate they dealt damage to the tarrasque dropped below the rate it regenerates. Then it spent all that time slowly losing the nonlethal damage, until finally it was enough to regain consciousness. The city is left in ruins, and now the nations of the world have to deal with the tarrasque acting like a roving natural disaster. Maybe at the end, you have a choice to rebuild Salt in the Wound and get that source of alchemical supplies back, or kill it for good as the only way to be sure this never happens again.

Later in that game, we abused gate spells to crash rocks into the Abyss at 80% the speed of light.

But that requires using real-life physics to figure out damage. It's better if you stick entirely to game physics, like the Locate City nuke.

Does anyone actually run an unmodified 5e tarrasque? Outside of a joke campaign?

The higher they are, the more fall damage. Of course, if there's enough of them, 30 feet may be enough.

Paladin: Slays the dragon

Bard: Lays the dragon

Minecraft Speedrunner: Yes

In D&D, your life generally is permanent. It's just that you'll only spend a little of it on the material plane.