Okay so that's what's causing the defense penalty...but why is he such a wuss? I set his tactics to Passive/Custom, and I have to devote one of his tactic slots to consuming a health poultice if his health drops below 25% or else he dies every fight.
Should I buff up his dexterity or constitution for awhile so he lasts longer or what?
The penalty to defense is probably from having powerful swings activated.
Sten looks tough but you need to adjust his tactics. I play on PC and had the same issue with him early on. His initial tactics set up had him activating threaten (which draws agro) automatically. Turn that off in tactics and you should see an improvement.
Also, he should never be your point man unless you have him in good gear.
Aggressive Self Health >50% use health poultice most powerful. Enemy target of controlled character mighty blow. Enemy target of controlled character sunder arms. Enemy target of controlled character etc....
Having a tactics slot for potions isn't a bad thing. I do no pause nightmare play throughs and all my chars have a potion slot. 2h is about ability spam. Once you get it, sunder armor should be in the top slot.
You shouldn't need to boost his dex and con is a terrible stat to boost. Just remember, even though he looks like a tank he isn't one. Need to manage his agro like he's a rogue really.
Per my above post make sure threaten isn't turned on or set to turn in automatically.