The Aux Subchaser is an extremely small boat that is almost impossible to hit with a torp, especially if it's maneuvering. Even if you do hit it with a torp, it probably won't sink for a couple of days.
My advice is to save your torps. Don't bother trying to take it out with the DG either. Very small target to try and hit, and your shells will probably just bounce off it as if you're hitting a steel wall. They're very small (unless I'm confused with a different boat) so, they really shouldn't carry all that many depth charges. Steal away, any way that you can. I actually think you can outrun it on the surface. Good luck mate.
Are you running silent? Have you deactivated your Damage Control Team?