Its veryy simpile yous somthing too drain his focus he will yous a spell to ragain his focus and when he youses the spell he will lose about 2.000 Hp keep going like this until he dies.sorry if i spell wrong.
psychological-you can use a focus drain. ultimatium is not recommended. biological-half of your bar must be fully upgraded disrupt. dont take leading strike if you have the perk which regains focus and hp over time. hydraulic-lucky you. you have more options. you can ice wall (max lvl) or stabilise repeatedly. just be sure to pack a few heals away for later.
It's a bit meaningless to attack on his life instead of focus. He has two ways to regen his focus, only one hurt on himself. The other is when he uses his light-attack. Watch out for that.
Well, for physiological, you really just need shock therapy (its the first one in the electric tree, its got a blue icon)to get rid of his re energizing buff which gives him focus. I'd love to have a epic long battle with the baron without him having any weaknesses and with attacks that aren't so deadly that your always on the defensive. A battle against someone with high health would be quite interesting right?