AJAX performance testing in LR

Hi I have to test AJAX based calls in Load Runner for performace.

Does any one have any strategy and the best way of getting these calls scripted in V-Ugen.

No differently than any other app

The approach should be the same as for any web-based application: VUGen will record the browser's web activity , whether it is AJAX or not

why is web_custom_request function generated and why is it used?

why is custom_request used and what is its importance?
when is this function generated?

It's basically the same as web_submit_data

Also refer to the documentation, of course.

web_custom_request is very similar to web_submit_data. There is a recording option that allows you to record all functions as web_custom_request instead of web_submit_data. VUGen will also use web_custom_request when it can't use other Web API functions, for example if a method besides POST or GET is used, etc.