GOT IT! ;-)
After an estimated 30-50 attempts (hard to say when you've stopped counting a *long* time ago), I just succeeded in getting past the Ox3 mission without cheating, auto tracking, wingmen etc. Just a Centurion with tachies and torps.
In case anyone is interested, here's the strategy that finally worked for me:
I set skill level to "Easier" (the lowest setting) for two reasons: a) The second wave *always* consists of 3 Demons, i.e. never 5. b) The Demons don't fly as fast (not using their burner?), allowing you to follow them on their runs away from the drayman.
At this level, I get past the first 3 *every* time. My kill strategy is simple: Tap the armor with my guns, *without* hitting the armor/hull (at least too much). Then, on a run away from the ship, kill off the Demon with a handful of torpedos. Rinse and repeat.
I should note that I use a mouse (not drag-mode, which is horribly buggy) and was having a few problems turning quickly enough because a fast turn speed requires moving the cursor way off center (and, at least with my somewhat laggy system, that made it a b*tch to get back to center in one attempt). Increasing the mouse_exponent value in the vegastrike.config file from 1.5 to 1.7 helped a lot. There's also a mouse_sensitivity setting, but the exponent setting seemed to work better for me.
Good Luck!
