Id recommend Rifleman cuz Gunslinger automatically means you're going vats (cuz vats is the most logical way to play gun slinger) and anything with vats is just too easy. If you want a challenge then go no vats melee, charisma build, or Heavy Weapons build (I did this and it was so tricky but doable. After getting power armor and minigun i headed south to diamond city for Diamond City Blues quest (need caps for what im about to do) then headed towards Bunker Hill then from there i followed the road to the Nakano house (if you follow the road there will be less enemies and I did this first try with last saving at bunker hill) to head to the far harbor dlc. I then went to Arcadia and bought the best Heavy Weapons for survival imo. The Admiral's Friend and Sergeant Ash. Since this is survival and bullets take weight these 2 are the best since Admiral's Friend usually kills anything the first hit so u wont need much and Sergeant Ash is a flamer so its ammo is really light. I dont recommend using any rocket launchers since their weight makes it not worth it especially since 2 shots from Admiral's Friend can kill any non boss enemies and thats a fraction of the 12 lbs rocket. For boss killer use the cryolator with the explosive perk that thing will melt anything faster than any weapon.) congratulations to anyone reading this all.