If they must run Gunslinger, set your tree perks around your exotic armour. Armour to consider are: Grasps, Spine,bones,celestial nighthawk. Dead eye is great with nighthawk in pve. Scavenger is great and incendiary grenades are good if you can't use tripmines well and having more knives is always good. Path unknown and way of the drifter with chain of woe for extra recovery. I found Gunslinger fun but I found it centred around single targets where Nightstalker has more crowd control abilities and more movement choices like keen scout with Graviton and vanish in smoke. Good luck, happy hunting.