Anubis said:2) Coup de Gras should kill unconscious regenerating creatures; Tarrasque can't be knocked out, and "instant death" attacks only take him down to -10.
Umm no. You can't CDG a troll, why should it work against the big T?
As far as killing it, your not supposed to. It IS the godzilla of D&D and always has been. It takes alot to kill it (very high level PCs and multiple wishes), but it can in theory be done.
As far as the CR, it is far too low. CR should be closer to 24-25 most likely. But then again its so stupid and without the ability to deal with flyers its not too much of a threat to most parties.
You can't kill it very well, but you can keep away without even really trying.