Do you have any links to builds which actually do so? I don't see how, for example, a spell allowing 1 critical hit a round with a bow is going to do more than 70 damage at very low levels.

But I really don't care if there are smarter ways of fighting the tarrasque. The point is the monk can fight it in accordance with the Marquess of Queensberry rules and has a good chance of coming out on top. It's a huge step up in power compared to a monk 19. Even if there are non-hitting ways of beating the monk, that's no reason to throw out balance for the hitting parts.

If you really must, imagine my super-duper homebrew creature: flying tarrasque. It flies, moves really really fast (10 000 feet per round, perfect manoeuvrability), and it can see you (+ a billion to spot, and blindsight, and it sees and hits things on the ethereal plane, and it can follow you through teleports and plane shifts and everything else, regardless of the time elapsed if it's busy), and it specifically wants to kill you and your party in particular. It's attacks and defences are exactly as strong as normal tarrasque. Does it seem reasonable that the monk can out-box it?

If you feel DR/20 is appropriate then a monk 19 is underpowered. I feel DR/20 is too much, and think monk 20 is overpowered. Either way, suddenly getting DR/20 is an anomalous jump.