It would help to know your Class, Perks, Stats and Equipment. Otherwise, we can only give generic advice, like the Heavy Slut Ray (and Stun Weapons in general) are absurd. Assuming you maxed out Aim/Physique.
I would suggest targeting the gunners first as they are the ones putting out most of the damage. They are weak to electic and corrosive damage and any way you can sunder the defenders armor will help take him down a lot quicker.
