In my opinion, in order to prepare for the GMAT, you should study the official guide thoroughly- even the answers that you got right. You must be aware that GMAT has two main sections: Quantitative and Verbal.
Here are some tips to study both sections:
Verbal:
Quantitative:
Getting into ISB:
As far as essays are concerned, this is one place you can definitely impress the admissions committee. You may have an average profile but through your essay, you can show that you can definitely fit well into the program and you have something unique to contribute. Show that you are committed to your reasons for doing an MBA.
Lastly, it is quite difficult to answer as to what profile the school is looking for. You cannot control the demands that the market has. If the demand for marketing consultants is high, then chances are that someone with a good marketing background could get in.
Just make sure that you have done enough to make up for your scores if you consider them to be weak. Other than this, ensure you do well on your GMAT.