Well... I just now managed to defeat him with my lvl 10, TankyTank Assault Rifle build. (A slightly experimental build, as I am going to max out Con to 16 as primary stat.) Normal Difficulty.
Carnifex had initiative, but I resisted his "yell"-intimidation attack, which is important. (If I dont resist, I will have my Gun skill reduced to almost nothing, meaning I almost cant hit him.)
I definately did not get him in first try, and I did need a little bit of luck, but because of my build I could fairly easily tank him for quite a long time. My weakest point was when he used Yell, and it became almost impossible for me to hit carnifex, even with a 9-bullet burst fire!
I relied mostly on damage resistance from armor, AND Damage Reduction.
A few vital things I learned/figured out in this regard, which others might find useful:
1. Dont use a metal armor, the moment Carnifex gets a few crippling strikes on you, your strength gets lower than the minimum strength of the metal armor, and your AP gets reduced by 15 points!. If you instead choose a good riot gear with reinforced fabric panel (and regenerative vest), you will actually get the same damage resistance against carnifex melee attacks, as you would with a Metal armor. Because Riot Gear has a 75% bonus to Damage Resistance and Threshold against melee! Only minus is the lower Armor penalty which makes Juggernaut Feat not work. (Riot Shield was NOT an option, as I was using Rifles)
2. Damage Reduction is not in itself overpowered, but when combined with Morphine, it gets pretty wild. With "Conditioning", "Lifting Belt" and "Mushroom Brew" I had 37% Damage Reduction, but when combined with Morphine I get a whooping 87% Damage Reduction, which are applied AFTER the armors Damage Resistance. When you multipliy that with the Damage Resistance of armor pieces, then Carnifex suddenly only damaged me for 10-15 damge per hit, which is much more survivable.
BUT... remember what happens at the end of the 3 rounds of morphine! (Actually that was how Carnifex died, I had him down on around 5% life, and then his morphine ran out taking away his last bit of health.)
Combine this with some Stun from a Taser, and a Flashbang, you can stay alive for a while.
Of the above, using the Riot Gear should be availible for most Character Builds, at least without the regenerative vest if you are not a crafter. And for stealth builds, it should be possible to craft a Riot Gear with Black overcoat that gives +stealth.
And by using a Lifting Belt and perhaps Mushroom Brew (if youre not a Psi-character), you can miximize the advantage of the Morphine. (Pure Morphine means 50% damage received, with just the belt and brew, you get that down to 30% damage received, which is almost doubling the advantage of the morphine.)
That was just my 2 cents in regards to Carnifex, at least sofar.
