What is the best way of filtering EEG signal for the specific frequency band?
Perhaps you should try some of the filter design tools that are available online.
You can define the required filter behavior (Cutoff frequency, Ripple ..) and you will get a design solution that fits your needs.
This kind of tools will help you know what is possible to achieve in terms of filtering performance given the constraints that you detailed
Checkout the following ones ( this is not an exhaustive list) :
http://t-filter.engineerjs.com/
https://webench.ti.com/filter-design-tool/filter-response
Best regards,
Mourad