Patience
 

If you get behind the destroyer escort and fire all of your torpedos into the convoy, dive deep and go ahead of them. You can hide out long enough that the Armed Trawler will start shooting and finally sink the Destroyer. It will fall back, you can surface and go for another round into the remaining Merchants. Leave the free camera on just so you can watch. Quite entertaining, the Destroyer just takes it until it sinks.