Basically, Get disrupt to level 2 and drain his focus like crazy. He will activate a buff that will restore his focus but hurt him for large damage, Keep doing this until he has low HP and he will eventually get the De-buff "Holy Scars" every once in a while.
You do know there are three different guides that cover this? Just look in the stickies. --
I just got through beating him w/ bio. Just use maxed out disrupt, which will deplete his focus, causing him to automatically refill it for around 2000-4000 health. After a while of doing this, he will automatically get the effect 'Holy Scars'. it lasts for two or three turns, and the baron takes up to around 4000 damage, depending on your level. Keep Veradeaux and Roald in Phalanx mode until the baron gets holy scars. Switch them to relentless when it is up, and back to phalanx when it is down, and you should be able to beat him.