Well, im in cust style and i have S ranked these damn shadows like 20 times and they still havn't given me a Shadow* and im not using navis or anything. I've been using lifesword PA's and varswords. And Longsword L, i only have 2 of them so i need one more to make my folder more fluid.
For LongSword L you need to S Rank Swordy viruses without Custom Style. Don't know why you can't get Shadow *s though. Make sure you're not firing your buster or anything (although that would be stupid for Shadows).
It might depend on where you're doing this at, for one thing. If Darkmanv3 is running around in undernet 6 Shadows will also appear there. I've had good luck getting Shadow * off them. However, I've only rarely gotten them off secret area shadows.
The Undernet 6 Shadows always appear with 1 other virus, but that Virus is usually in the first row, so if you hit a lifesword draw on turn 1 you can win the battle in less than a second.