Just noticed a funny thing.
Monks of the Open Hand get a fun little bonus whenever they hit with their Flurry of Blows:
- It must succeed on a Dexterity saving throw or be knocked prone.
Note how there's no mention of creature size here. A Monk's DC is 8 + prof bonus + wisdom mod. Assuming 20 wis, the level 20 DC for the Dex Saving Throw is 19. The previewed tarrasque has a Dex saving throw of 0, which means 90% of the time, a monk can knock a tarrasque on their back with the touch of their hand. Of course, the tarrasque can just shove it off... but that seems like a pretty silly usage of Legendary Resistance.
A slightly more applicable usage of this would be knocking flying monsters out of the sky. The Ancient Red Dragon has a Dex saving throw of +7, so you knock an ancient red dragon out of the sky 55% of the time. (A flying creature knocked prone falls).
Now, how does a monk fly up to meet his enemy in the sky? Well, they could use an external resource like a wizard... or you could just be a Monk of the Four Elements. They can Ride the Wind (cast fly) and then use Water Whip to literally whip a dragon out of the sky (knocking prone). Best part is you don't need to get through AC... it's exclusively a dex saving throw.
Suddenly, I'm imagining a great monastery where monks train to fight giant titans and literally bring them down out of the sky. A tiny flying gnome monk whipping an Ancient Dragon's flight sky gives me the giggles.