The topics from Mathematics under General Aptitude are - Sets & Relations and Mathematical Reasoning.
Based on the NATA - 2017 question paper, the General aptitude section had 4 questions from Sets & Relations and there were no questions from Mathematical reasoning.
the exam pattern of NATA was changed last year, that's why there will be only two questions consisting of 40 marks each.
Mostly two percpective drawings and one 3Dcomposition OR one percpective and one 3D and one sculpture
If u have good drawing skills and if U r good in aptitude and mathematics u can do it easily
The difficulty level cannot be known for sure since there has been only one question paper(2017) on the revised syllabus so far. But as for the preparation, you can refer the NCERT - Exemplar Mathematics(11 + 12) books. Also the NATA - Mock Tests on edge incorporate the same difficulty as the last year paper.