The FIITJEE AITS (All India Test Series) is, as the name suggests, a series of tests designed to test students on all topics of JEE Advanced after completion of the syllabus.
Being a ex-FIITJEE student, I can tell you that the AITS is atleast five times more than difficult than the actual exam. My chemistry teacher told the entire class not to take the result of AITS too seriously. He further added even he couldn't solve some of the questions which were asked in the AITS.
To answer your question, yes, the AITS is as tough as they say, if not more.