This fight starts with an air fight. First, shoot his ship down. When you get to the fist fight, wait until he throws a big uppercut. Hit him while he is stunned. When he gets to two thirds of his health, he will send out guards to defeat you. If you got hurt, look for health icons that they drop. When you continue the fight against the Baron, he will have a second attack. When he growls and gets angry, he will jump on the wing of the plane. Jump to avoid this attack. When he gets to one third of his health, he will send out more guards. When you defeat them and the fight continues, the guards will still be coming. Use the guard that sprays oil and ignites the oil against the Baron. Lure him to the oil and keep him there until he is set on fire.
Reuben Reye