Firebomb

You load firebombs into a catapult when the base across the field is still made of wood. The jugs burst on landing and the fuel burns, so wooden walls, doors and barricades lose health while anyone defending that spot has to back away from the fire.

That second part matters more than people expect. A couple of bombs on a wooden compound gate, or on the roof the defenders are shooting from, clears them out long enough for the rest of your group to move up. You can also place one by hand at the door you want gone and set it off by damaging it, though a placed bomb hits weaker than a launched one.

Against stone and metal the fire does very little. Switch to the propane explosive bomb for those walls, and use scattershot when the targets are people in the open rather than buildings. Keep an eye on fuel too: every bomb eats low grade fuel, and early in a wipe that same fuel has to keep your lanterns and other lights going.