IFoS and CSE have the same prelim examination. This explains the high focus on environment related topics in prelim examination.
Thus if you prepare environment part thoroughly you’ll have an edge. Moreover if you have decided to give IFoS seriously – and have taken Forestry as one of the optional – then you should go through the entire syllabus of it once before prelims. There is a chance that there’ll be 2-3 questions which a Forestry student can answer easily compared to a general candidate.
Also dont leave ancient and medieval history section. TN board class 11th book is a savior in this regard.
For current affairs – refer to at least two sources – Vision and Insight make for a good combo but you can rely on others too.
Next practice as many mock test as you can. They’ll help in incremental improvement. Join a test series and give test at test center if you can. Nothing matches the actual experience of giving the test.
IFoS prelims cutoff is usually 15-20 marks above the CSE cutoff. It was 132-133 for IFoS – 2016 while 116 for CSE 2016. Your aim should be to optimize your marks by increasing the accuracy. Attempting more could backfire in this uncertain era of UPSC.