For those who are struggling to reach a C2 level in Spanish
For those who are struggling to reach a C2 level in Spanish, I highly recommend "Schaum's Outline of Spanish Grammar, 6th Edition", Advanced Spanish Step-by-Step by Barbara Bregstein, Barron's Educational Books, El cronometro / The Timer, or "Gramática de uso del Español. Teoria y practica. Con solucionario. C1-C2".
As a bilingual speaker, all of these books cover important higher level Spanish topics such as all of the 13(yes 13!) verb tenses, the situations where you use those verb tenses, and irregular adjectives, verbs, and adverbs.