Backing up data in luns

yes, install agent on the server where the LUN is mounted and backup the data from there else use ndmp feature in Backup Exec to backup the NAS. Refer Software compatibility list for the BE version and also https://www.veritas.com/support/en_US/article.100017787.html

J_Huggins 

If my understanding is correct, the NAS lun is mounted/mapped on the Windows server as a Network Drive, right? 
In Windows explorer, drive letter E will have a network icon on the left with the network path in brackets (\\filer\path).
Is this correct? 
If so, you cannot backup drive letter E. You can follow this TN to 'Configure share level backup' :
https://www.veritas.com/content/support/en_US/article.100008169.html

Gurvinder has answered question no. 2 - If your NAS make/model is supported for NDMP backup (and you have correct license), you can backup entire NAS. 
Instructions for NDMP config in BE Admin Guide.