To kill the Baron, get his Focus down to 0 (using Disrupt if you're biological, Traumatize as psychological) and he will regen his focus to 100 and take about 2500 damage for that turn and the next 2. When you get him down to about 15-20k health, he will take massive extra damage for a few turns (the debuff is Holy Scars) so do as much damage as you can. If you don't take him down by then, which you probably won't, he will once again take extra damage, and you can finish him off.
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