Despite the diversity among groups of asian americans and asian pacific islanders, they have been treated as a(n) group for generations.

The answer to this question is the "monolithic group generations". When we say monolithic it is directly mean a big or a societal or a religion or an organization. A group of people with different goals and belief. Thus to complete the sentence we have it despite the diversity among different groups of Asian Americans and the Asian Pacific Islanders, they have been treated as a monolithic group of generations.