As mentioned by several people, I picked up the melee weapons dropped by the enemies. One was an Epic bat (my first Epic weapon at the time) and using charged melee attacks with it one-shot most enemies and allowed me to kill the legendary boss with five or six hits.
I'm playing on hard, rest of game has been ok so far, just this one mission i had found to be impossible. Thanks for tips guys, sorry i work during week hadn't had a chance to play or check out thsi thread (took me a while to find it, not easy to locate your threads)