Yes, the BHU UET for BSc in agriculture is tough because the seats are limited. Being a popular course, the cutoff is also high. A few tips that will take you a long way are:
Download BSc AG Syllabus to make a strategy to cover the topics systematically.
Solve previous year exams
Focus on NCERT
Subject wise, Chemistry and Biology was mainly from NCERT. Also, focus more on the botany section in biology.