The phrase "defy easy classification" is correct and usable in written English.
You can use it when you want to express that something is difficult to categorize or classify into a distinct category or group. For example: "The style of music he created defied easy classification; it blended elements of both jazz and classical music."
Editor Harold Ross sometimes referred to his pieces, which tended to defy easy classification, as "highlife lowlife" stories.
While the best-known Korean movies of the past few years are stylish, violent genre works — crime thrillers like "Oldboy" or effects-heavy fantasies like "The Host" — Mr. Lee's films have a more subdued, literary flavor, and they tend to defy easy classification.
Grant Gee's "Patience (After Sebald)" is, to some degree, a survey of the work of the German writer W. G. Sebald, who spent most of his career in England and whose books, blending fiction, memoir, philosophy and travel writing, defy easy classification.
Michael Winterbottom has proven he can work in many genres: film noirs (The Killer Inside Me), buddy comedies (The Trip and The Trip to Italy), and some movies that defy easy classification.
Painstakingly crafted to defy easy classification, The Lobster is the superbly original English-language debut by Greek writer-director Yorgos Lanthimos (Dogtooth, Alps).
Rather, those factors that are known or suspected to contribute to the clinical picture of spinal pain and all are considered in each patient, with the recognition that many patients have a variety of factors involved in his or her clinical picture, and thus defy easy classification.
The end result is a stylistic complexity in African art that defies easy classification.
Although Ethiopia's complex relief defies easy classification, five topographic features are discernible.
We have spent our lives caught in a kind of demographic and cultural void, defying easy classification, denied even the community-building cliché of a generational identity.
Fleeting musical dedications to friends and artistic figures are a trademark of Mr. Kurtag, an important composer whose music, influenced by Bartok and Webern, defies easy classification.
Her music certainly defies easy classification, and has somehow – you might even say miraculously – remained immune to the conventional demands of record labels.
