Hello,
Yes, it is definitely possible to get 90% in nios senior secondary with hard work and effort.
The following points must be remembered:
For business studies revise everyday as the subject is volatile and you may forget.
Accountancy- practice maximum number of problems from different books and of different types
Economics- practice different problems and remember all the formulas. Keep revising the formulas so that you do not forget.
Do not forget to practice last 5 years question papers as a lot of problems of the same type may come.
Do not lose hope. This is definitely an achievable target and you can achieve it by hard work.
All the best.
Hope this helps.