How tough is IIM B test for getting into fellowship program at IIM Bangalore?

Unlike CAT, RAT (Research Aptitude Test) is not a unified exam. Evert IIM conducts its own admission process for FPM and thus the pattern of RAT varies from IIM to IIM. One of the patterns is termed as minor RAT followed at IIMB and at a few other IIMs. The RAT consists of different types of CAT questions including questions on verbal skills, logical reasoning, quantitative etc. However, the quantity of questions is less and the questions are distributed differently. Questions are designed to determine minimum eligibility requirements and are not meant for elimination thus the overall test is not too difficult.

It's important to note that the RAT pattern for each of the IIMs changes every year. Though some preparation definitely needs to be made for these exams, I would not suggest giving more time to prepare for interviews than the written exam.