How do you kill ground troops with a starfighter?

I don't think there's any simple trick, just a few strategies that can help, and getting good at those strategies can sometimes give you that 30/2 K/D spread you're looking for. Here are the strategies that have helped me get better at using ships for something other than just dogfighting:

• Choose your view. Some pilots prefer the cockpit view for targeting purposes. Some prefer the third-person camera for a bigger field of view. Try both and see what feels more comfortable.
• Strafing runs seem to work better for me than dive-bombs. As you have perceptively noted in your post, down is a pretty bad direction to be flying. Maybe try a more gradual dive, varying your speed to find a comfort level that keeps you from being a sitting duck, as well as keeping you from zipping past your objective (or, more often, into the ground.
• Some ships have missiles, which can be fired without locking onto a target. With a little practice, you can use them as a straight-line projectile. (Interesting note, the Ion rocket launcher, which only locks on to vehicles, can be used to hurt ground troops if theyre caught standing between you and your vehicle target).
• Some maps are more conducive to air-to-ground combat. I find Hoth to be the easiest for seeing and attacking ground troops, either way, it's all about your comfort level navigating the map, and gaining a familiarity with its features and choke points.

All in all, I don't think there is a tried-and-true method or quick answer (unless I've missed it as well). I think it's mainly about practice and finding your own play style (even within the arena of air-to-ground combat). But, as I'm sure you know, pulling your throttle back, sighting up on a map's common choke-point from a long distance away, and unleashing the fury of your blasters until you cover that long distance will undoubtedly serve you well. To be consistent with it will most likely just take a lot of practice, and a lot of rage-inducing crashes.