Can some one tell me the best practice to do dynamic CSS mostly less-css

It depends on your requirements.
1) You can use percentage values in many cases which negate the need for complex solutions.
2) Find the break points in your site and work from that with media queries, you should only need a few, if not you may want to restructure the site.

Thanks,

So it seems that for responsive design it is media queries so.

Best Regards