There's lots of good tactics spread around here if you search for them, and there are tactics in the GWX manual, and I think Brag has some on his web page. In a nutshell, with GWX 2.1 you should be coming in submerged, slow/quiet (2 knots or less), and deeper as the seas are calmer. Avoid coming within 500-1000M of the lead escort if at all possible (that seems to usually result in automatic detection). Once they're towards your stern (so you're in their baffles/undetection zone) start moving to periscope depth. Once as PD, start shooting!
The other possibility is early in the war, go in a periscope depth, duck and dodge the pursuing destroyer, and attack any how!
