How did tigerclaw lose his last life in warrior cats?

I think you mean Tigerstar, since he died after he became leader.

In The Darkest Hour, when Tigerstar orders BloodClan to attack, Scourge becomes angry and tells Tigerstar that he (Scourge) is the only cat to lead BloodClan, and that he has decided they won't. Tigerstar gets mad and attacks Scourge, who slices Tigerstar's belly from his throat to the base of his tail, which was such a vicious and serious wound that Tigerstar lost all nine of his lives at once.

~cqpkytty