I seem to have found a reliable solution for destroyers. (Though not recommended. My tactics seem to be more aggressive than a lot of the sub skippers I see on here. Besides, the speed and maneuverability of a destroyer make for a poor target and tend to waste fish.) So far, have successfully sank 14 destroyers using this method, and would be good to note that I have only encountered 14 destroyers on my patrols
I set my course to have a 0 degree bearing on my target. When he commences his attack run (points his bow straight at you, pings with active sonar, and starts zig-zagging), I completely clear the TDC, set first to fire at a 0 degree bearing, next 2 (in rapid succession), at 5 degree left and right bearings, and the next 2 at 8 degree right and left bearings. The destroyer can not as it seems, outmaneuver that particular spread of fish. It seems thus far as a good destroyer tactic. (Though please keep in mind that it requires 5 torpedoes, not counting the possibility of duds, run-deeps, etc, and that I personally recommend not attacking a destroyer unless the situation is absolutely perfect for you and you have not been detected. This method I will note is for use when you have been detected and the destroyer is making his attack run) (also to note, the best range for this particular spread is between (1200 and 900 yards)