Is everyone complaining of one-shot NPCs playing on Legendary Strongholds?
I use a 6 Red build and don't get one-shot on Heroic. I used a tankier build on Legendary Summit and didn't get one-shot.
Also, if the issue is the game being too hard, can't you just turn down the difficulty level? When I run time trials I can facetank nearly everything and never have to reload my Chatterbox, since it's on hard.
Want to feel like a God? Play Hard.
Want to cruise, but have minor risk? Play Challenging.
Want to use the game Mechanics and some thought? Play Heroic.
Want to try-hard and get punished for mistakes? Play Legendary.
OP is using a face tank build with Glass Cannon. Not surprised it's a disaster.
My most effective solo PvE build is Negotiators with Glass Cannon and Vigalence, but I make liberal use of decoys and turrets, and stay in cover, or funnel Legendary enemies to choke points. That's for missions where I know the spawn points well. For open world, I switch to Obliterate or go skill.
It does no good to report cheaters because they don't do anything about it. Their reporting system Sucks! Came across a cheater in the DZ went toe to toe with him just to have him pull the jitter routine/ side scroll bit. Thats what you can do when NPC have abilities that you have cheat to be able to do.