Believe me. Its near impossible with GWX.
I did the torpedo mission for the hell of it a few days ago and though I managed to get them all but one, my last two torpedoes were duds. And the other small merchant that I hit with one torp didn't sink.
You can do it but its hard.
Then theres the mock convoy battle. Theres only one Escort but its light out, you only have steamers, you're 800meters from the convoy, and you only have 120 meters to dive. Maybe with an electric torp you could get away with sinking the escort but its hard to escape him in the shallows. And since he's around 1500 meters away to begin with you're boned.