Since for last half a decade, at least, SSC has been asking arithmetic questions that are slightly above the level. Also, just solving the questions won't help you unless you start learning the concepts and their application.
Since you are comfortable with the questions in R.S. Aggarwal, I can infer that you have learned enough of the topic to apply its concepts directly. The problem arises when direct application is not an option. For that you need to develop a better understanding of the concepts. That will only come when you are less interested in solving the questions through formulae and more interested in understanding how that formula has been developed. You might come up with your shortcuts in the process.
Learning basic formulas is a good idea but rote learning all the shortcuts without understanding at the concepts behind those shortcuts is a recipe for disaster. It's like going to a new city and trying to apply all the shortcut routes that a local is using without getting used to the safer main route. One wrong turn and you'll be lost (I know we have Google Maps these days, so this analogy doesn't really work!).
So, my suggestion to you would be to learn the concepts first without thinking about the questions or what is going to be asked in the exam. Get used to the topic and only then jump onto the questions. If you are getting stuck solving tricky questions, analyse why you are not able to apply the concepts you learned into the questions that you are solving. The trick is to understand the concept well enough to solve these tricky questions.
Any other remedy will only be short-term and poses a risk to your aim of cracking the SSC CGL exam.