You don't have to use any of this. No opponent will destroy an oil barrel or throw it at you.
I had multiple enemies in my PTs go out of their ways to interact with barrels to create surface/explosions, whatever it's by using special arrows/bombs to hit them or their weapons (and kill themselves in the process for the last one usually).
Throw has only been done by the large enemies and the barrels have to be in their path-range for them to pick them up. Put an oil barrel near the 3 Ogres in the Village and they will pick it up and throw it at you once you start combat.