I'm reading the Tarrasque entry in the Hypertext D20 Open Source (here) and I can't help but wonder how exactly can it be considered easy to take on. I'm not even saying "a joke". That is just a manner of saying that it's not that impossible.
The bastard has almost a 1000 HP, better initiative than most PCs, and each and every attack he does is almost an automatic hit with insane amounts of damage.

A DC 36 Will save is what you have to pass not to pee your pants the momet you see / hear him and he's pretty much immune to everything...
Possibly because I've never run a game above the 10th level (for the PCs) and challenges were proportional, but I fail to understand how the Big T is a Joke. Le alone for the fact you don't just find his lair by following rumors, since there is no one left alive to tell them, even in a simple pit fight, cages open and release the Tarrasque and 4 high level PCs into a 300 yards wide arena, with time to buff up with all the possible casting and summoning, beating the Tarrasque should still make it an epic, once-in-a-1000-years accomplishment. He's literally Godzilla from Inoshiro Honda's imagination. Size matters, they say, and also the fact that it took 12 Maverick Air-to-Ground missiles dead on the side to take the lizard down.

So, unless we are talking about a group of four 20th level PC with proportional equipment, which would still be "moderate difficulty" and not a joke, the Tarrasque rules. Also because in my power scale four 20th level PCs = the Gold Saints from Athens descended from the Great Tample and decided to save mankind from extinction.

O.