I managed to kill Kellogg without dying horribly because Survival difficulty makes everything a billion times more deadly in addition to neutering your carrying capacity, fast travel, and saves. I miss Hardcore Mode.
To elaborate, you get into a short conversation with the guy that ends the same way every time, with you being forced to murder the shit out of him in incredibly Bethesda-like fashion, at which point his synths start blasting you and he pops a Stealth Boy like a prick. You can't target him in V.A.T.S. while he's invisible, unlike both previous 3D games (even if you couldn't actually use it to fire at any of those targets), so you have to either fire blindly at where he could be while his lackeys are unloading lasers into your everywhere or chuck a Molotov or grenade or something and hope it hits him without immediately killing yourself. Because it's Survival difficulty, where ONE Molotov or grenade or something can kill you just as easily as it kills mooks. Cannot tell you how many times I've mistimed a throw and smashed a bottle of flaming death into cover immediately adjacent to my face, or screamed in horror as an enemy threw one of their own at me and I died with full health.
And Kellogg has about seventy billion HP, and can Stimpak himself, and has a gun that kills you in frighteningly few hits. So I ended up saying "Fuck this", abandoning my pretenses of playing through things The Way They Should GoTM (aka without cheesy bullshit) in the process, and sneak-attack-headshotted him with a five-times-cranked laser musket before he even started talking so the fucker couldn't just heal and run away. His lackeys were laughably easy with the big man down.
I love modes that add needs and diseases and stuff like this to make things feel a little less gamey but man does Survival push my damn buttons sometimes.
(Slightly Modded) Fallout 4