Abhijit,
Just recollected that I have run a similar report and the best way which comes to my mind is to run a Top N report from USM against a group of devices . When running a top N report you actually have an option to set a threshold for multiple metrics .
For example you want to get a list of devices which are being under utilised in both the CPU and Memory , select both the variables in filters and set a threshold of say less than 30% . This will give you a list of devices under utilised for over a month or a week with the QoS metrics of your choice in %
This is the easiest way to generate such report without using UR which comes to my mind. Hope this helps