Defeating the Baron is easiest with Psychological. If his focus is 15 or lower, you must use Echoed Screams (fully upgraded), which makes Baron very weak against Shadow Damage for 2 turns. . He'll use Deep Burning, doing massive damage to himself for 3 turns. Next turn, use Implosion (fully upgraded).
Deep Burning = 2,500 damage Deep Burning + Echoed Screams = 4,000 damage Deep Burning + Echoed Screams + Implosion = 24,000 damage!
More tips:
1. When Baron uses Electrical Storm, he gains 20 Focus each turn. Get rid of that status effect quickly by using Shock Therapy.
2. When Baron uses Holy Scars, take advantage of it by putting your team into Aggressive Mode.
If you use sacrifice at the beginning, then you don't have to learn any tips. You don't need shock therapy and don't have to worry about putting your team into aggressive mode!