Do system wise for 1-2 systems then do subject wise for 1 -2 subjects . See where you more comfortable and where you are more faster.
Studying better is different for each one of us.
You have too seek what works for you. Usually it is a trial and test method.
Be objective . Set an aim. Then work towards that aim.
Logic always prevails . As a rule.
The more simple your approach the more productive your effort .
The more complicated your approach the less productive will be your efforts.