For Narvik, I got a lot of useful advice last year here:
http://www.subsim.com/radioroom/showthread.php?t=176911
I always try to aim for Narvik fjord and set up shop on April 9. You get a pack of British destroyers coming in and out early on the 10th, and then Warspite (called something else in the game) and a pack of destroyers on the 13th.
But you will want to to ask the medic to tighten up your sphincter an extra notch, because you will be fighting in the worst possible GWX environment. 20m depth under keel, if you are lucky, and the fjord is, if I remember, only about 4K wide at the best choke-point.
My tactic is to take on the April 10 destroyers on their way out (don't get greedy, 1 will do). And then take on Warspite on her way in. I'd say I sink her at least half the time, and that I survive 30% of the time.
The nooks and crannies of the fjord are your best friends, as are electric eels (forget the gas eels).
Good hunting!