The horses here aren't dead yet, right?

Well, 4e had the solution all along (yes, I'm referring to my old post on Reddit)

For those of us whose horses are alive, can someone loop us in on this one?

I homebrew a composite tarrasque with abilities from all editions… and I’ve found that I don’t even need the earth binding aura to bring ranged attackers down. All you need is the reflective carapace, so missed attacks bounce back and hit someone else instead, and maybe give the big T a nice sized jump so it can simply leap up into the air to catch fliers like a dog catching a frisbee.

But even the jump may not be necessary if you remember that tarrasques burrow like bullettes (land sharks), and the party probably needs to, for example, protect a city from being eaten, not just kill it in a field. So they have to come close just to deal with the burrowing.

And don’t forget to give the tarrasque its nigh-immortality back. Bringing it to 0 hp shouldn’t kill it. You need to bring it to 0 and then cast wish.