Or if you have a limited supply of gunpowder but a way to make sufficient numbers of appropriately shaped projectiles, an air rifle would do the job and is much lighter and simpler than a trebuchet.
Isn't it true that you can never generate a supersonic projectile with just air pressure?
I don't believe that is true - rockets produce a very supersonic airflow from gas pressure.
I looked into it. Yes, you're correct, I was wrong. Although there is a speed limit of mach 5 (theoretical, not reachable in practice!) for shockwaves in normal atmosphere.